HVAC Handbook Part 6 Air Handling Equipment

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fans

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air conditioning apparatus

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unitary equipment

3

accessory equipment

4

SYSTEM DESIGN MANUAL
SUMMARY OF PART SIX
This part of the System Design Manual presents practical
data and examples for selection and application of air handling
equipment for normal air conditioning systems.
The text of this Manual is offered as a general guide for
the use of industry and of consulting engineers in designing systems.
Judgment is required for application to specific installation, and
Carrier is not responsible for any uses made of this text.

INDEX

Part 10. Air Handling Equipment | Chapter 1. Fans

CHAPTER 1. FANS
This chapter presents information to guide the
engineer in the practical application of fans used in air
conditioning systems.
A fan is a device used to produce a flow of air. Use of
the term is limited by definition to devices producing
pressure differentials of less than 28 in. wg at sea level.
TYPES OF FANS
Fan are identified by two general groups:
1. Centrifugal, in which the air flows radially thru the
impeller. Centrifugal fans are classified
according to wheel blading ; forward – curved,
backward–curved and radial (straight).
2. Axial flow, in which the air flows axially thru the
impeller. Axial flow fans are classified as
propeller (disc), tubeaxial and vaneaxial.
Figures 1, 2a, 2b and 2 c show the various types
of commonly applied fans.

The centrifugal fan is used in most comfort
applications because of its wide range of quiet, efficient
operation at comparatively high pressures. In addition,
the centrifugal fan inlet can be readily attached to an
apparatus of large cross – section while the discharge is
easily connected to relatively small ducts. Air flow can be
varied to match air distribution system requirements by
simple adjustments to the fan drive or control devices.

APPLICATION
When a duct system is needed in an air conditioning
application, a tubeaxial, vaneaxial or centrifugal fan may
be used. Where there is no duct system and little
resistance to air flow, a propeller fan can be applied.
However, self – contained equipment often utilizes
centrifugal fans for applications without ductwork.

Fig. 1 – Centrifugal Fan

Fig. 2 – Axial Flow Fans

Part 10. Air Handling Equipment | Chapter 1. Fans

Fig. 3 – Sound Power Levels
Axial flow fans are excellent for large air volume
applications where higher noise levels are of secondary.
They are, therefore, often used for industrial air
conditioning and ventilation. These high velocity fans
require guide vanes to obtain the best efficiencies when
operation against pressures considered normal for
centrifugal fan . However, these fans may be applied
without guide vanes.
Figure 3 illustrates the approximate sound power
level of a typical centrifugal fan and an axial flow fan. The
frequencies detectable by the human ear ( 300 to 10,000
cycles per second ) are the least favorable for the axial
flow fan. Therefore, to obtain acceptable sound levels
with the axial flow fan, sound attenuation may be
required.

The concept of specific speed is useful in describing
the applications of various fan types. Specific speed is a
fan performance index based on the fan speed, capacity
and static pressure. Figure 4 shows the ranges of
specific speed in which six types of centrifugal and axial
flow fans operate at high static efficiencies. This figure
indicates that forward – curved blade centrifugal fans
attain their peak efficiencies at low speeds, low
capacities and at high static pressures. However,
propeller fans reach high efficiency at high speeds and
capacities and at low static pressures.
The horsepower characteristics of the various fans
are such that a type may be overloading or non
overloading. The backward – curved blade centrifugal fan
is a nonoverloading type. The forward – curved and radial
blade centrifugal fans may overload. Axial flow fans may
be either nonoverloading or overloading.
All fan types may be utilized for exhaust service. Wall
fans operate against little or no resistance and therefore
are usually of the propeller type. Propeller fans are
sometimes incorporated into factory – built penthouses or
roof caps. Hooded exhaust fans and central station
exhaust fans are typically of the centrifugal type. Axial
fans may be suitable for exhaust applications, particularly
in factory installations.

STANDARDS AND CODES
Fan application and installation should conform to all
codes, laws and regulations applying at the job site.
The AMCA standard Test Code for Air Moving
Devices, Bulletin 210, prescribes methods of testing fans,
while AMCA rating standards prescribe methods of
rating.

CENTRIFUGAL FANS
Centrifugal fans are identified by the curvature of the
blade tip. The forward – curved blade curves in the
direction of rotation ( Fig. 5a ) . The radial blade has no
curvature (Fig. 5b). The backward–curved blade tip
inclines backward, curving away from the direction of
rotation ( Fig 5c ) . The curvature of the blade tip defines
the shape of the horsepower and static pressure curves.
The characteristics of the three main types of
centrifugal fans are listed in Table 1.

Fig. 4 – Specific Speed Ranges

FORWARD – CURVED BLADE FAN
A typical performance of a forward – curved blade
fan is shown in Fig. 6 . The pressure rises from 100 %
free delivery toward no delivery with a characteristic dip
at low capacities. Horsepower increases continuously
with increasing air quantity.

Part 10. Air Handling Equipment | Chapter 1. Fans

BACKWARD – CURVED BLADE FAN
A typical performance of a backward – curved blade
fan is shown in Fig. 7. The pressure rises constantly from
100 % free delivery to nearly no delivery. There is no dip
in the curve. The horsepower curve peaks at high
capacities. Therefore, a motor selected to satisfy the
maximum power demand at a given fan speed does not
overload at any point on the curve, providing this speed
is maintained.
Two modifications of the backward – curved blade
fan are the airfoil and backward – inclined blade fans.

Fig. 5 – Fan Blades

Fig. 6 – Forward-Curved Blade Fan Performance

These are illustrated in Fig. 5 d and 5 e . Both are
nonoverloading types.
The airfoil blade fan is a high efficiency fan because
its aerodynamically shaped blades permit smoother
airflow thru the wheel. It is normally used for high
capacity, high pressure applications where power
savings may outweigh its higher first cost. Since the
efficiency characteristic of an airfoil blade fan usually
peaks more sharply than those of other types, greater
care is required in its selection and application to a
particular duty.
The backward – inclined blade fan must be selected
closer to free delivery; therefore, it does not have as

Fig. 7 - Backward-Curved Blade Fan Performance

Part 10. Air Handling Equipment | Chapter 1. Fans

Fig. 8 – Radial Blade Fan Performance
great a range of high efficiency operation as does the
backward –curved blade fan. Manufacture of an inclined
blade is understandably a simpler operation.
Fig. 10 – Vaneaxial Fan
RADIAL BLADE FAN
Typical performance of a radial (straight) blade fan is
shown in Fig. 8. The pressure characteristic is continuous
at all capacities. Horsepower rises with increasing air
quantity in an almost directly proportional relation. Thus,
with this type of fan the motor may be overloaded as free
air delivery is delivery is approached.
The radial blade fan has efficiency, speed and
capacity characteristics that are midway between the
forward–curved and backward–curved blade fans. It is
seldom used in air conditioning applications because it
lacks an optimum characteristic.

AXIAL FLOW FANS
Figure 9 shows a performance characteristic typical
of a propeller fan.
The tubeaxial fan is a common axial flow fan in a

Fig. 9 – Propeller Fan Performance

tubular housing but without inlet guide vanes. The blade
shape may be flat or curved, of single or double
thickness.
The axial flow fan has become particularly associated
with the vaneaxial type which has guide vanes before or
after the fan wheel. To make more effective use of the
guide vanes, the fan wheel usually has curved blades of
single or double thickness. Figure 10 is a sectional view
of the vaneaxial fan.
The curved stationary diffuser vanes are the type
most frequently used when higher efficiency vaneaxial
fans are desired. The purpose of these vanes is to
recover a portion of the energy of the tangentially
accelerated air.
Typical performance of an axial flow fan is shown in
Fig.11.

Fig. 11 – Axial Flow Fan Performance

Part 10. Air Handling Equipment | Chapter 1. Fans

FAN DESIGNATION
CLASS OF CONSTRUCTION
The AMCA has developed standards of centrifugal
fan construction based on the pressure the fans are
required to develop. The four classes of fans appear in
Table 2 , Each of the various fan manufacturers has
defined his own maximum wheel tip speed for each class.
The required fan class can be determined from Chart
1 it outlet velocity and static pressure are know.
Calculation of velocity pressure and total pressure is thus
eliminated. This chart is based on standard air ( 29.92 in.
Hg barometric pressure and 70 F temperature ).
If nonstandard conditions of temperature and altitude
are encountered in an application, the calculated static
pressure should be corrected before entering Chart 1.
This procedure is described in the section entitled Fan
Selection. See Example 3.
Minimum first costs can often be achieved by using a
larger size fan of a given class than by choosing a
smaller fan size of a higher class. If a selection lies on the
border line, both alternatives should be considered.

Some manufacturers offer packaged fans and motors
which are not defined in terms of classes. These
packages are made of Class I or II parts, modified slightly
to hold the motor within the fan base. The fan package is
less expensive than the equivalent Class I or II fan and is
satisfactory for most applications. Packaged fans are also
offered in construction lighter than Class I. Manufacturers’
specifications usually distinguish between light and heavy
construction.
A pressure class standard pertaining to centrifugal
fans mounted in cabinets has also been published by
AMCA. Cabinet fans are commonly used with central
station fan – coil equipment. The three classes of such
fans are defined in Table 3 .

CHART 1 – CONSTRUCTION CLASS PRESSURE LIMITS

Part 10. Air Handling Equipment | Chapter 1. Fans

Fan class nomenclature does not apply to fans used in
fan–coil terminal units where the manufacturer limits such
fans to a particular maximum speed and static pressure.
FAN ARRANGEMENTS
Centrifugal fan drive arrangement , standardized by
AMCA , refers to the relation of the fan wheel to the
bearings and the number of fan inlets. Figure 12 indicates
the various arrangements.

The fan drive may be direct of by belt. With the
exception of packaged fans and motors, direct drive is
seldom employed in air conditioning applications
because of the greater flexibility afforded by the belt
drive.
Arrangements 1 , 2 and 3 are commonly used for air
conditioning. The remaining choices are modified
versions of Arrangements 1 and 3 . Double inlet fans for
belt drive are available in Arrangement 3 and 7.
In selecting a suitable fan arrangement first cost and
space requirements are considered. Single inlet while
double inlet fans are lower in cost in the large sizes. For
the same capacity a single inlet fan is about 30 % taller
than the double inlet type, but only about 70 % as wide.

Part 10. Air Handling Equipment | Chapter 1. Fans

Arrangement 3 is the most widely used because the
bearing location eliminates the necessity for a bearing
platform. Cost and required space is therefore minimized.
For single inlet applications Arrangements 1 and 2
are used where the fan wheel is less than 27 inches in
diameter. Arrangement 3 is not used since the bearing on
the inlet side is large enough, relative to the inlet area, to
affect fan performance. Fans of larger sizes and double
inlet fans are not limited in this way.
Arrangement 1 is usually more costly than
Arrangement 2 because it has two bearings and a base.
Where Class III construction is required, Arrangement 1 is
preferred over Arrangement 2.
If Arrangement 3 is to be used at air temperatures
exceeding 200 F or Arrangement 1 or 2 at temperatures
exceeding 300 F, the fan manufacturer should be
consulted so that the proper bearing or heat slinger can
be specified.
Table 4 compares the costs of fan and drive for
several single inlet arrangements. Selections are based
on a constant air quantity and static pressure.

Fig. 13 – Motor Positions

Figure 13 shows the motor positions possible for a
belt – driven fan. Use of Positions W and Z results in the
simplest construction of fan base and belt guard.
Figure 14 shows the standard rotation and discharge
combinations available.

Fig. 14 – Rotation and Discharge

Part 10. Air Handling Equipment | Chapter 1. Fans

Axial flow fans are available for belt drive or direct
connections. Therefore, two arrangements have been
standardized throughout the industry, Arrangement 4 is
driven directly, Since the motor is in the air stream, the
application of this arrangement is limited to the handling
of air which will not damage the motor. Arrangement 9 is
belt – driven, with the motor located outside of the air
stream and the drive protected.

FAN PERFORMANCE
Fan performance curves show the relation of
pressure, power input and fan efficiency for a desired
range of air volumes. This relation is based on constant
speed and air density.
Static rather than total pressure and efficiency are
usually inferred. Static pressure best represents the
pressure useful in overcoming resistance. However, static
pressure is less applicable where the fan outlet velocity is
high. Further, if the fan operates against no resistance,
static pressure is meaningless. In these cases total
mechanical efficiency is used.
Fan performance may be expressed as percentages
of rated quantities or in terms of absolute quantities. The
former method is illustrated in Fig. 6 , 7 , 8 , 9 and 11.

LAWS OF FAN PERFORMANCE
Fan laws are used to predict fan performance under
changing operating condition or fan size.
They are applicable to all types of fans.
The fan laws are stated in Table 5. The symbols
used in the formulas represent the following quantities:
Q
– Volume late of flow thru the fan.
N
– Rotational speed of the impeller.
P
– Pressure developed by the fan,
either static or total.
Hp
– Horsepower input to the fan.
D
– Fan wheel diameter. The fan size
number may be used if it is
proportional to the wheel diameter.
W
– Air density, varying directly as the
barometric pressure and inversely as
the absolute temperature.
In addition to the restrictions noted in Table 5,
application of these laws is limited to cases where fans
are geometrically similar

Part 10. Air Handling Equipment | Chapter 1. Fans

and where there is no change in the point of rating on the
performance curves. Because of the latter qualification,
fan efficiencies are assumed constant.
Geometrically similar fans are those in which all
dimensions are proportional to fan wheel diameter.
The same point of rating for two fans of different size
means that for each fan the pressure and air volume at
the point of rating are the same fraction of shutoff
pressure and volume at free delivery, provided the
rotational speed is the same in either case. For example,
an operating point on Fig.7 will not change with the
application of laws 7 thru 9 , even though specific values
will change.
Example 1 – use of Laws 1 thru 3
Given:
Air quantity
- 33,120 cfm
Static pressure
- 1.5 in wg
Fan speed
- 382 rpm
Brake horsepower
- 10.5
Find:
Capacity, static pressure and horsepower if the speed is
incased to 440 rpm.
Solution:
Capacity
= 33,120 x (440 / 382 ) = 38,150 cfm
Static pressure
= 1.5 x ( 440 / 382 ) 2 = 2.0 in. wg
Horsepower
= 10.5 x ( 440 / 382 ) 3= 16.1 bhp

FAN CURVE CONSTRUCTION
Fan performance is usually presented in tabular form
(Table 6). However, for a graphic analysis performance
curves are more convenient to use. If no curves are
available, tabular values of pressure and horsepower may
be plotted at constant speeds over the given range of
capacities, The resulting curves may then be used as
described under Fan Performance in a System.

FAN PERFORMANCE IN A SYSTEM
SYSTEM BALANCE
Any air handling system consists of a particular
combination of ductwork, heaters, filters, dehumidifiers
and other components. Each system therefore has an
individual pressure–volume characteristic which is
independent of the fan applied to the system. This
relation may be expressed graphically on a coordinate
system identical to that of a fan performance curve. A
typical system characteristic is shown in Fig. 5.
System curves are based on the law which states
that the resistance to air flow (static pressure) of a system
varies as the square of the air volume flowing thru the
system. In practice a static pressure is calculated as
carefully as possible for a given system at the required air
quantity. This establishes one point of the system curve.
The remaining points are obtained by calculation from the
above law, rather than by further static pressure
calculations at other air quantities.
When a fan performance curve for a given fan size
and speed is superimposed upon a system characteristic
as in Fig. 15 , there is the only possible operating point
under the conditions. If the fan speed is in decreased, the
point of operation moves upward toward the right. If the
speed is decreased, the operating point moves down and
the to the left.
Figure 15 illustrates the effect on system
performance of operation at other than design conditions.
Such a situation could be caused by dirty filters, wet coil
versus dry coil operation of the dehumidifier, or the
modulation of a damper. Lines of constant brake
horsepower have been included for ease of analysis.

Part 10. Air Handling Equipment | Chapter 1. Fans

Fig. 15 – Effect of Change in Design Conditions
Example 2 describes the analysis.
Point O is the design point. Points A and B are new
operating points resulting respectively from an increase
or decrease in system resistance. Point A and B are
single points each of two new system characteristics.
Example 2 – Operation Above Design Static Pressure
Given:
Air quantity
- 3300 cfm
Static pressure
- 1.5 in. wg
Fan speed
- 1000 rpm
Fan performance
- Fig . 15
Find:
Air quantity, static pressure and brake horsepower if the
resistance of the filters is 0.15 in. wg greater than estimated for
design.
Solution:
1. From design point O ( Fig. 15 ) rise vertically to point Y at
a static pressure of 1.65 in . wg ( 1.5 + 0.15 ) and 3300
cfm.
2. Since the fan operates at 1000 rmp, proceed to the 1000
rpm fan curve along a line parallel to the system
characteristic. At the new operating point A the fan
delivers to the system, 3175 cfm at a static pressure of
1.5 in. wg. The required power input at the new
conditions is 1.4 bhp.

PRESSURE CONSIDERATIONS
An analysis similar to that of Example 2 indicates that
overestimating the static pressure increases the required
horsepower. Operation in this case is at point B of Fig. 15
, rather than at point O . Therefore, the addition of a safety
factor to the calculated static pressure tends to increase
fan horsepower requirements unnecessarily. The static
pressure used to select a fan should be that pressure
calculated for the system at the design air quantity.
If the static pressure is overestimated, the amount of
increase in horsepower and air volume depends upon the

steepness of the fan curves in the area of selection. Fig.
16 shows that volume deviations may be large if the fan
curve is relatively flat. With a steep pressure
characteristic, pressure differences may have little effect
on air volume and horsepower. For this reason a fan with
a steep performance curve is well suited to a system
requiring an air volume relatively independent of changes
in system resistance. An example of such a system is an
induction unit primary air system.
Conversely, a variable volume system requires a
pressure nearly constant with changes in air volume.
Thus, a fan with a comparatively flat pressure
characteristic is more appropriate.
STABILITY
Fan operation is stable if it remains unchanged after
a slight temporary disturbance or if a slight permanent
disturbance produces only a small shift in the operating
point.
Instability is a surging or pulsation which may occur
when the system characteristic curve intersects the fan
curve at two or more points. This is rare occurrence in a
single fan. When two or more forward curved fans are
connected in parallel, It is possible that the composite
curve have an unstable area such as shown in Fig. 17 . If
the operating point falls in this area, either adding or
subtracting resistance allows operation at a stable point
on either side of this area. When operation occurs such
that only one sharp intersection of fan curve and system
curve is possible, there is no condition of instability.
System resonance is rare thing but may occur in
systems utilizing high pressure fans with a duct system
turned to a particular frequency like an organ pipe. With
operation to the left of the pressure peak, a pressure
increase is accompanied by a capacity increase, in turn
tending to further increase pressure. This condition may
be altering the system characteristic curve so that the
operating point falls between the pressure peak and the
free delivery point.

Part 10. Air Handling Equipment | Chapter 1. Fans

FAN SELECTION
The system requirements which influence the
selection of a fan are air quantity, static pressure, air
density if other than standard , prevailing sound level or
the use of the space served, available space, and the
nature of the load. When these requirements are known,
the selection of a fan for air conditioning usually involves
choosing the most inexpensive combination of size and
class of construction with an acceptable sound level and
efficiency.
Outlet velocity cannot be used as a criterion of
selection from the standpoint of sound generation. The
best sound characteristics are obtained at maximum fan
efficiency. Fans operating at higher static pressures have
greater allowable outlet velocities since maximum
efficiency occurs at higher air quantities. Thus, any limits
imposed on outlet velocity in relation to sound level
depend upon the static pressure in addition to ambient
sound levels and the use of the area served. In regard to
sound generation a fan should be selected as near to
maximum efficiency as is possible and adjacent ductwork
should be properly designed, as described in Part 2 .
The best balance of first cost and fan efficiency
usually results with a fan selection slightly smaller than
that representing the maximum efficiency available.
However, selection of a larger, more efficient fan may be
justified in the case of long operating hours. Also, a larger
fan may be economically preferable if a smaller selection
necessitates a larger motor, drive and starter, or heavier
construction.
The selection of a fan and drive can affect
psychrometric conditions in the area served. If the
combination produces an air quantity below that required

at design conditions, the resulting room dry - bulb
temperature is higher. When the air quantity is greater
than required at design conditions, room controls prevent
a fall in temperature.
ATMOSPHERIC CORRECTIONS
Fan sound level does not vary sufficiently with
altitude to warrant using sound ratings at conditions other
than sea level.
Fan tables and curves are based on air at standard
atmospheric conditions of 70 F and 29.92 in. Hg
barometric pressure. If fan is to operate at nonstandard
conditions, the selection procedure must include a
correction. With a given capacity and static pressure at
operating conditions the adjustments are made as
follows:
1. Obtain the air density ratio from Chart. 2
2. Calculate the equivalent static pressure by
dividing the given static pressure by the air
density ratio.
3. Enter the fan tables at the given capacity and
equivalent static pressure to obtain speed and
brake horsepower. This speed is correct as
determined.
4. Multiply the tabular brake horsepower by the air
density ratio to find the brake horsepower at the
operating conditions.
If atmospheric corrections are ignored in the fan
selection, fan speed and air capacity may be too small,
and the brake horsepower somewhat high.
Example 3 illustrates a fan selection at high altitude.

CHART 2 – ATMOSPHERIC CORRECTIONS

Fig. 17 – System Instability

Part 10. Air Handling Equipment | Chapter 1. Fans

Example 3 – Fan Selection at High Altitude
Given:
Air quantity*
- 37,380 cfm
Static pressure* -1.45 in. wg
Altitude
- 5000 ft
Air temperature - 70 F
Fan ratings
- Table 6
Find:
Fan speed, brake horsepower and class.
Solution:
1. From Chart 2 the air density ratio is 0.83.
2. The equivalent static pressure is equal to 1.45 /
0.83 or 1.75 in. wg.
3. Select from Table 6 a size 7 double inlet fan,
operating at 520 rpm and requiring 13.45 bhp.
4. The design speed at 5000 ft is 520 rpm.
5. The brake horsepower for the less dense air at
5000 ft is 0.83 x 13.45 or 11.2 bhp.
6. At the fan outlet velocity of 1800 fpm and the
equivalent static pressure of 1.75 in. wg, enter
Chart 1. The selection is well within the range of
a Class I fan. This is the proper selection.
At altitudes above 3300 feet, fan motor temperature
guarantees may not apply. High altitude applications
should therefore be brought to the manufacturer’s
attention.
ACCESSORIES
Fan accessories are available to fulfill specific needs.
Where applicable, the following accessories can aid in
assuring a satisfactory air conditioning system.
Access Doors
Access doors on the fan scroll sheet should be
provided whenever there is a possibility of dirt collecting
the fan.
Drains
A drain should be specified whenever condensation
or water carry – over may occur.

Fig. 18 – Variable Inlet Vanes
Outlet Dampers
Outlet dampers are a volume control device and may
be automatically or manually actuated. They may be used
for applications requiring extreme capacity reduction for
short periods of time or for small adjustments. These
dampers are illustrated in Fig. 19. Their use is further
discussed under Control.
Isolators
In order of decreasing vibration isolation efficiency,
steel spring isolators, double rubber – in shear isolators,
and single rubber – in shear isolators are all used for fan
installations. These isolators are normally used in
conjunction with steel channel bases so that the fan and
the motor may be mounted on an integral surface. For a
more complete discussion of vibration isolation, refer to
Chapter 2 of this part.
Bearings
Ball bearings are the most common type of bearing
used on fans. The sleeve oil bearing can be provided at
an extra cost and is initially a quieter bearing. However,
its quietness has been overemphasized since the noise
does not materially add to the fan air noise.

Variable Inlet Vanes
Figure 18 shown a set of variable inlet vanes. These
vanes are a volume control device and may be
automatically or manually actuated. They are
recommended for applications with long periods of
reduced capacity operation and for use with static
pressure regulators. Use of variable inlet vanes is further
discussed under Control.

Fig. 19 – Outlet Dampers

Part 10. Air Handling Equipment | Chapter 1. Fans

CONTROL
Variation of the air volume delivered by a fan may be
accomplished by several methods:
1. Variable speed motor control
2. Outlet damper control
3. Variable inlet vane control
4. Scroll volume control
5. Fan drive change
Use of a variable speed motor to control fan capacity
is the most efficient means of control and the best from
the standpoint of sound level. However, it is the most
expensive method.

Use of outlet dampers with a constant speed motor is
the least expensive method but the least expensive
method but the least efficient of the first three mentioned
above.
Variable inlet vanes may be used to adjust the fan
delivery efficiently over a wide range. This method
controls the amount of air spin at the fan inlet, thus
controlling the static pressure and horsepower
requirement at a given fan speed.
Figure 20 compares variable inlet vane control, outlet
damper control and speed control as each affects fan
performance. The horsepower curves indicate the power
required at various vane settings, damper positions and
fan speeds respectively.

Part 10. Air Handling Equipment | Chapter 1. Fans

The horsepower curve of variable inlet vane control
(Fig. 20) is based on a fan designed with supplementary
fixed air inlet vanes, such that there is no loss in efficiency
when variable vanes are used instead. A loss of static
efficiency as great as 10% results from the use of variable
inlet vanes on a fan designed with an open inlet.
Tubeaxial and vaneaxial fans are often equipped with
adjustable for matching the fan to fan to system
requirements.
Propeller fans may be speed–controlled or blade–
adjustable.

LOCATION
Refer to Part 2 for the aspects of fan location. The
effect of fan motor location on the system cooling load
and air volume is discussed inn Part 1.

MULTIPLE INSTALLATIONS
Fans may be arranged in series or in parallel to
provide for operating not met by the use of a single fan.
Possible series applications include:
1. Recirculating fan
2. Booster fan
3. Return air fan
A recirculating fan increases the supply air to a
space without increasing the primary air (Fig. 21) . The
purpose is to obtain greater air motion, usually in
relatively lightly loaded area, or to decrease the
temperature difference between supply air and room air.
An industrial application prompted by the former purpose
is the recirculation of air in an inspection room served by
the same system as a neighboring production area.
A booster fan is used the step up the static pressure
in a distribution system in order to serve a remote area,
loaded intermittently; when this area is loaded, it requires

Fig. 21 – Recirculating Fan

Fig. 22 – Booster Fan
a small air quantity relative to the total primary air (Fig.
22) . A conference room (space A) could be conditioned
in this manner.
The most common series application is the return air
fan, usually used on extensive duct systems to facilitate
the controlling of the mixture of return air and outdoor air
and to avoid the excessive room static pressures
required (Fig. 23) . Use of a return air fan also provides a
convenient method for exhausting air from a tightly
constructed building.
In air conditioning, fans are seldom directly staged,
with the outlet of the first being the inlet of the second.
The fan efficiency and the operating economy suffers if
this method is used for merely obtaining a higher static
pressure.

Fig. 23 – Return Air Fan

Part 10. Air Handling Equipment | Chapter 1. Fans

Fans may be applied in parallel if dictated by space
limitations or if provision is to be made for a future
addition. Centrifugal fans are available factory – mounted
in cabinets for the former reason. Parallel fans provide
greater capacity at a common static pressure.

However, a parallel design is seldom chosen just to
increase capacity since no improvement in fan efficiency
occurs and economy is not warranted by the greater first
cost of the parallel installation.

Part 10. Air Handling Equipment | Chapter 2. Air Conditioning Apparatus

CHAPTER 2. AIR CONDITIONING APPARATUS
This chapter contains practical information to guide
the engineer in the application, selection and installation
of various types of air conditioning apparatus, remote
from the source of refrigeration.
Although the concept of air conditioning includes the
moving, heating and cleaning of air, this chapter is
devoted primarily to cooling, dehumidifying and
humidifying equipment. Other types of air handling
equipment are discussed in Chapters 1 and 3 of this part.

TYPES OF APPARATUS
Air conditioning apparatus may be classified into two
major groups:
1. Coil equipment in which the conditioning
medium treats the air thru a closed heat transfer
surface.
2. Washer equipment in which the conditioning
medium contacts the air directly.

These two groups may be subclassified as shown
Chart 3.
Because of its specialized application, packaged or
unitary air conditioning equipment is described in
Chapter 3 of this part. Terminal equipment is discussed in
Parts 10 and 11.

STANDARDS AND CODES
The application and installation of air conditioning
apparatus should conform to all codes, laws and
regulations applying at the job site.
Applicable provisions of the American Standard
Safety Code B 9.1 and ARI , ASHRAE and AMCA
Standards govern the testing, rating and manufacture of
air conditioning apparatus.

FAN–COIL EQUIPMENT
As the term implies, the primary constituents of a
fan – coil unit are a fan to produce a flow of air and a
chilled water or direct expansion coil to cool and
dehumidify the air.

Part 10. Air Handling Equipment | Chapter 2. Air Conditioning Apparatus

Fig. 26 – Spray Coil Section

Fig. 24 – Single Zone Fan- Coil Unit

Fig. 25 – Multi-Zone Fan-Coil Unit

Accessories such as a heating coil, a humidifier and a
filter section are normally available to perform, if
necessary, the remaining air conditioning functions. The
required components may be assembled into a factory –
fabricated, cabinet style package. Figures 24 and 25
show respectively a single zone and a multi–zone
fan – coil unit.
A spray coil section is shown in Fig. 26. Since such
equipment in intended for incorporation in a built up
apparatus, it is not fan–coil equipment. However,
because of the similarity of function, spray coil equipment
is discussed in this section. Differences in application
and layout will be noted as they exist.

Fig. 27 – Spray Fan-Coil Unit

Part 10. Air Handling Equipment | Chapter 2. Air Conditioning Apparatus

Figure 27 illustrates a spray fan – coil unit.
Single zone and multi–zone fan–coil units differ
physically in the location of the fan relative to the cooling
coil. In a single zone unit, the fan is located downstream
of the cooling coil. Therefore, this type of unit is often
termed a “draw–thru” unit. A multi-zone unit may be
referred to as a “blow–thru” unit since the fan is located
upstream of the coil . Figures 28 and 29 indicate the flow
of air thru the two types of central station fan–coil
apparatus.
Typical variations occurring in total pressure, static
pressure and velocity pressure, as air passes thru a fan –
coil unit, are illustrated in Fig. 30 and 31. The use of a fan
equipped with a diffuser helps to convert velocity
pressure to static pressure with a minimum energy loss.
Fan–coil units are furnished with either forward or
backward–curved blades. Forward-curved blade fans are
well suited for such use, since they perform at slower
speeds than other types of fans. Fan wheel construction
is lighter in weight, more compact and less expensive
than with backward–curved blades. Longer fan shafts are
permissible because of the slower speeds.

APPLICATION
The application of air conditioning equipment is
influenced by the cooling load characteristics of the area
to be served and the degree of temperature and humidity
control required.
A single zone unit most effectively serves an area
characterzied by a relatively constant or uniformly varying
load. Ideally, this are would be a single large room.
However, multi–room applications are practical, provided
a given variation in load occurs in all rooms
simultaneously and in the same proportion. If required,
zoning may be accomplished by reheat or air volume
control in the branch ducts.
In a multi–room application where load components
vary independently and as a function of time, a multi–
zone apparatus provides individual space control with a
single fan unit. For this type of load a multi–zone
installation is less expensive than a single zone
installation with a multiplicity of duct reheat coils.
Since a multi–zone unit permits outdoor air to bypass
the cooling coil at partial loads, its use is particularly
adapted to applications with high sensible heat factors
and minimum of outdoor air. If humidity control is required
with a multi–zone unit, a precooling coil may be installed
in the minimum outdoor air duct.

Part 10. Air Handling Equipment | Chapter 2. Air Conditioning Apparatus

A standard fan-coil unit affords a close temperature
control only. A measure of humidity control may be
obtained by providing a unit humidifier such as a city
water spray package. However, if more certain humidity
control is required, a spray coil section or spray fan-coil
unit is better suited to the application.
Spray coil equipment may be utilized for summer
cooling and dehumidification, winter humidification and
evaporative cooling in intermediate seasons. Its use is
preferred for applications where humidity control is
required, such as in industrial processes, hospitals,
libraries and museums. Spray coil equipment may also
be equipped with a spray water heater to provide
simultaneous cooling or heating and humidification.
Standard fan-coil equipment, both single zone and
multi-zone, may be obtained for air deliveries as high as
50,000 cfm. Multiple spray coil sections are available for
air quantities exceeding 60,000 cfm. Where the size of
available factory-fabricated equipment is exceeded the
apparatus must be constructed of individual cooling coils
or spray coil sections.
Static pressure limitations on fan-coil unit fans vary
widely with the manufacturer considered. Available
cabinet fan pressure classes are defined in Chapter 1 of
this part.
UNIT SELECTION
The selection of fan-coil equipment is a matter of
achieving the required performance at the maximum
economy. The economic aspect includes not only the
particular unit and coil chosen but also the effect of that
choice on other system components, such as piping,
ductwork and refrigeration equipment.
The selection procedure involves choosing the unit
size and the coil. A coil selection includes the
determination of the coil depth in rows required, the
0ptimum coil fin spacing and, in the case of chilled water
coils, the appropriate circuiting.
Unit Size
With the dehumidified air quantity known, the choice
of unit size normally precedes the coil selection. In most
cases, the size is determined by the cooling coil face
velocity.
When selecting a coil face velocity, it is suggested
that the highest allowable face velocity he used in the
interest of economy. Manufacturers rate their (coils at
maximum face velocities proven by tests to he
satisfactory, with respect to both the entrainment of
moisture droplets and air resistance. However, if
simultaneous reheat and dehumidification is required of
the unit, the maximum recommended cooling coil face
velocity may be less than that otherwise allowed,

depending on the design of the particular unit in question.
Since a unit reheat coil is not as deep as the cooling
coil and does not condense moisture, limiting the unit size
by limiting the heating coil face velocity is not
economically justifiable. Manufacturers of fan-coil
equipment have design their internal heating coils to
provide optimum performance at recommended cooling
coil face velocities.
As explained in Chapter 1 of this part, fan outlet
velocity should not be used as a selection criterion
reflecting the intensity of sound generation. Sound
characteristics improve with rising fan efficiency, rather
than with decreased outlet velocities.
Coil Selection
A particular cooling coil is selected to produce a
desired effect on the air passed thru it, in accordance
with the sensible, latent and total cooling loads calculated
for the space and with the condition of the air entering the
coil. However, the final selection defines also the required
chilled water flow, the pressure drop at that flow and the
required entering water temperature; or in the case of a
direct expansion coil, the refrigerant temperature.
Therefore, the coil selection should be made with regard
to refrigerant side or chilled water side performance as
well as to air side performance.
Thus, each coil selection has two facets which may
be regarded as independent for the purposes of
selection. Air side and refrigerant side performance
should be considered separately and then matched to
produce the final economically optimum coil selection.
The apparatus dewpoint method of coil selection
provides means for matching air side and refrigerant side
performances. This method is described in Part 1.
The two-step concept of coil selection is presented
as follows:
1. Make a tentative coil selection in terms of rows
and fin spacing, based on the bypass factor required by established air conditions. Coil bypass
factor determines apparatus dewpoint.
2. Determine the refrigerant side performance,
using the apparatus dewpoint found in the first
step. This involves finding the required refrigerant temperature for direct expansion coils or
the chilled water quantity, temperature and
resulting pressure drop for water coils.
Thus, the coil can be tentatively selected without
regard to the final refrigeration machine selection. If the
first. coil selection does not provide satisfactory
refrigerant side performance, another coil with adequate
air side performance may be tried. The optimum selection
assures proper performance at the least owning and
operating cost.

Part 10. Air Handling Equipment | Chapter 2. Air Conditioning Apparatus

Often in a multi-zone application, the apparatus
dewpoints of the various areas differ. Rather than
penalizing the cost of the entire system by selecting the
lowest room apparatus dewpoint as the coil apparatus
dewpoint, a higher, more representative apparatus
dewpoint may be chosen, and a compromise accepted in
the design relative humidity in the room with tl1e lower
apparatus dewpoint. The increased relative humidity is
offset by a decrease in dry-bulb temperature. Such a
decision may be required in the case of a conference
room, with its relatively high latent load. If a compromise
is unacceptable for this application, maximum economy
may be achieved by furnishing the special area with a
separate system.
The various types of coil ratings and selection
techniques encountered either use directly, or are
derived from, one of two methods. They are the
apparatus dewpoint (effective surface temperature)
method and the modified basic data method. The latter
involves calculating coil performance from basic heat
transfer data and equations. It combines air side and
refrigerant side performance determination into a single
operation. However, the basic data method requires
assumptions which are usually modified later in the
selection, and is therefore a trial-and-error procedure.
Calculated coil depth may be a decimal figure which
must be rounded to a whole number, in turn necessitating
a recalculation of performance. The apparatus dewpoint
method is derived from the two-step concept of coil
selection arid implements its use. Coil rows are dealt with
in terms of standard whole numbers only.
Charts 4 and 5 are conversion charts used to
evaluate the air side performance of any cooling coil, with
entering and leaving air conditions established. This
performance is in terms of coil bypass factor and
apparatus dewpoint. A straight edge, fixed at the entering
dry-bulb temperature and rotated to pass thru the various
intersections of the coil bypass factor and the line
connecting entering and leaving wet-bulb temperatures,

indicates the coil bypass factor which satisfies the leaving
dry-bulb temperature. The apparatus dewpoint can he
read at the chosen intersection.
Where the bypass factor for a particular coil is
unknown, the coil performance may he plotted on the
chart, and the bypass factor may he read at the
intersection of the entering-leaving wet-bulb and dry-bulb
lines. The bypass factors of various coils may thus be
directly compared.
When selecting a cooling coil in conjunction with an
air conditioning load estimate form, the bypass factor of
the coil selected should agree reasonably with the
bypass factor assumed in the estimate. If it does not, the
estimate should be adjusted accordingly, as indicated in
Part 1.
Refrigerant side coil ratings presume a tentative coil
selection when based on the apparatus dewpoint. Chart
6 and Table 7 illustrate apparatus dewpoint refrigerant
side ratings for chilled water and direct expansion coils
respectively. Such charts are used in the second step of
the two-step approach described above.
Table 8 shows the entering wet-bulb type of rating for
direct expansion coils. This method of presentation is
used frequently and may or may not be derived from the
apparatus dewpoint method.
For a direct expansion coil, optimum coil circuiting is
incorporated by the manufacturer into the coil design. A
direct expansion coil experiences a decreased capacity
with an increased refrigerant pressure drop caused by a
greater coil circuit length. This is true even with a given
coil surface.
Chilled water coils are usually offered with two or more
circuiting arrangements, and the final coil selection
prescribes the circuiting. The coil with the least number of
circuits has the greatest number of passes back and forth
across the coil face and vice versa. The minimum
circuited coil has a greater capacity and produces a
higher chilled water temperature rise at a given water
quantity.

Part 10. Air Handling Equipment | Chapter 2. Air Conditioning Apparatus

Part 10. Air Handling Equipment | Chapter 2. Air Conditioning Apparatus

Part 10. Air Handling Equipment | Chapter 2. Air Conditioning Apparatus

Part 10. Air Handling Equipment | Chapter 2. Air Conditioning Apparatus

However, the greater number of passes of a minimum
circuited coil results in a pressure drop higher than that
thru a coil of the same size but with more circuits and less
passes. Minimum circuited coils are often used on large
extensive systems in which the greater pumping head
required is more than offset economically by the reduced
first cost of piping and insulation.
With the required air side coil performance given, the
greater the difference between apparatus dew-point and
entering chilled water temperature the smaller the
required water quantity will be. Therefore, the choice of a
chilled water temperature may involve an economic
analysis of the first costs and operating costs of the
refrigeration plant versus the cost of the piping system.
The selection of the water temperature should not be
arbitrary; however, experience has shown that a
temperature approximately 5 degrees below the
apparatus dewpoint is the maximum water temperature
that should be used to effect an economical system
design. If the resulting water quantities seem to be too
high , a lower temperature can be assumed, and its

influence on the refrigeration machine size, power input
and piping costs should be studied. With a given coil,
load and apparatus dewpoint, when the chilled water
temperature is reduced , the required water quantity
decreases and the temperature rise increases.
By using a coil which requires a smaller water
quantity at a higher temperature rise, the following
advantages may be realized:
1. a. A smaller refrigeration machine may be
selected, or
b. The horsepower requirement may be
reduced for the same size machine by
operating at an increased evaporator
temperature, or
c. The condenser piping or heat rejection
equipment may be reduced for the same
size machine by operating at a higher
condensing temperature with less
condenser water.
2. Lower chilled water distribution costs with saving
in piping, pump and insulation may be obtained.

Part 10. Air Handling Equipment | Chapter 2. Air Conditioning Apparatus

A limitation is also imposed on the minimum chilled
water quantity by the velocity required for efficient heat
transfer. A minimum Reynolds number of 3500 is
suggested toinsure predictable and efficient performance
of a coil. The minimum chilled water flow required to
maintain this Reynolds number is approximately 0.9 gpm
per circuit for a 5/8 in. OD coil tube diameter. For a 1/2 in
OD tube diameter, the minimum flow suggested is 0.7
gpm per circuit.
Well water may be circulated thru chilled water coils
if it is sufficient quantity and at a satisfactory temperature.
However, well water temperature are usually low enough
to produce sensible cooling only and little or no latent
heat removal. In such a case, the well water may be
utilized in a precooling coil to remove some of the
sensible heat. The remaining cooling load ,sensible and
latent , is handled by supplement refrigeration.
Manufacturer’s recommendations regarding maximum and minimum direct expansion coil loadings should
be followed. Selections at loadings below the minimum
may result in unsatisfactory oil return, poor refrigerant
distribution and coil frosting.
Atmospheric Corrections
Cooling coil ratings are based upon the standard
atmospheric conditions of 29.92 in. Hg barometric
pressure. For atmospheric pressures significantly
different, such as at altitudes exceeding 2500 feet, a
correction should be applied to the air quantity before
making the coil selection.
Assuming that the necessary corrections have been
made to the load calculation and the sensible heat factor
as described in Part 1, the following procedure should be
applied to the unit selection:
1. Obtain the density ratio from Chapter 1, Chart 2.
2. Multiply the calculated dehumidified air quantity
by the density ratio to determine the equivalent
air flow at sea level.
3. Use this adjusted air quantity, together with the
calculated cooling load and sea level refrigerant
side coil ratings, to determine the coil water flow
and pressure drop or refrigerant temperature.
The calculated dehumidified air quantity is used with
no correction to determine unit size and coil face velocity.
However, the coil air side pressure drop must he
corrected, as described in Part 2.
Fan performance is analyzed in Chapter 1 of this part
and motor selection is influenced as outlined in Part 8.

ACCESSORIES
Heating Coils
Unit heating coils for fan-coil equipment are available
in a variety of depth and fin spacing combinations and in

both the nonfreeze steam and U-bend types. The latter
type may be used with hot water or steam and may be
obtained in different combinations of tube face and fin
spacing to produce different rises with the same entering
air temperature, face velocity, and steam pressure or hot
water temperature. Heating coils are also usually capable
of being mounted before or after the cooling coil.
During the intermediate season, any zone served by
a multi-zone unit should be able to obtain heating or
cooling on demand. Since there is no common supply air
duct in which air mixing can occur on a multi-zone
installation, the problem of air temperature stratification is
of considerable importance. Stratification across the unit
heating coil may cause some zones to be denied heat
when it is needed.
The throttling of a steam control valve may produce
stratification if the steam condenses fully before reaching
the end of the tube or circuit. Therefore, it is suggested
that full steam pressure he applied to a multi-zone unit
heating coil whenever heating may be required by any
zone.
In order to provide an air path of approximately equal
pressure drop thin either of the two widely differing beat
transfer surfaces in a multi-zone fan-coil unit, perforated
balancing plates are often used. It may be necessary for
the engineer to select such a device, particularly if no
heating coil is required.
The application and selection of heating coils is
described in detail in Chapter 4 of this part.
Humidifiers
On a fan-coil unit not equipped with recirculated
water sprays, humidification may be obtained by means
of a city water spray humidifier, a steam pan humidifier, a
steam grid humidifier or a humidifying pack. Spray coil
and steam grid equipment provides the most effective
humidity control.
A city water spray humidifier consists of a header,
spray nozzles and strainer. Either atomizing or nonatomizing sprays are available. The latter type requires a
lower water pressure. In either case, the spray density or
amount of water circulated per square foot of cooling coil
face area is considerably less than that of a recirculated
spray coil. Therefore, although lower in first cost, the city
water spray humidifier is less efficient than a recirculated
spray coil. An eliminator is not usually required for a city
water spray.
The use of copper fins on copper tubes with spray
humidifiers is suggested when city water has a specific
electrical conductance of 500 or more micromhos at 77 F.
(See Part 5 for values of water conductance in various
locations.) Aluminum fins may be used if city water is of
the proper quality. The use of copper fins should be

Part 10. Air Handling Equipment | Chapter 2. Air Conditioning Apparatus

considered where industrial gases such as hydrogen
sulphide, sulphur dioxide or carbon dioxide are present
and where salty atmospheres prevail.
If air flow is not maintained thru a spray section when
it is operating, wetting of the unit and leakage may result.
Therefore, a solenoid valve should be installed or other
suitable precautions taken to stop the sprays when the
unit fan is not running. To maintain a minimum coil air flow
when utilizing face and bypass dampers, a minimum
closure device should be provided on the face dampers.
The spraying of heating coils may result in scaling on
the coil and the production of odors. This practice should
therefore be avoided.
The use of spray humidifiers with a multi-zone unit
should be avoided. Since the coil is subjected to a
positive static pressure, spray water may leak from the
unit cabinet. If sprays are used, they should be of the
atomizing type. A grid or pan humidifier is preferred for
this usage.
Grid humidifiers are lengths of perforated steam
piping wrapped with wicking such as asbestos. The pipe
is mounted in an open pan, pitched to facilitate
condensate drainage. The condensate drain line from the
unit should be trapped to provide a water seal, as
described in Part 3. Steam pressures should not exceed
5 psig for this application, and the steam used should be
free of odors.
The mixing of steam with conditioned air normally
produces a negligible increase in the air dry-bulb
temperature. This type of humidification, therefore,
approximates a vertical line on a psychrometric chart.
Figure 32 illustrates the process. When designing a
system using a grid humidifier, the temperature of the air
entering the humidifier should be high enough to permit a
moisture content at saturation (point C) equal to or
greater than the desired air moisture content.
Pan humidifiers include a pan to hold water, a steam
coil to evaporate the water, and a float valve for water
make-up. For this application, a steam pressure of 20
psig is suggested for maximum humidifying efficiency.
Humidifying packs use a fill (often of glass fibers) as an
evaporative surface. The pack is located in the air stream
and water is sprayed over the fill.
Spray Water Heaters
Spray coil equipment may be provided with a spray
water heater to permit simultaneous cooling or heating
and humidification. Such flexibility is required during
winter operation or where the volume of outdoor air is
large in relation to the total air quantity. Typical
applications include certain industrial processes or
hospital operating rooms. These processes are described

in Part 1.
Face and Bypass Dampers
On applications employing face and bypass control
of coil equipment, the fan selection anti air distribution
system should be based on an air quantity 10% above
the design dehumidified air volume. This additional air
quantity compensates for leakage thru a fully closed
bypass damper and for air quantity variations occurring
when face and bypass dampers are in an intermediate
position. With the bypass dampers fully open, system
static pressure may be reduced and air quantity and fan
brake horsepower increased. Therefore, on face and by
applications especially, fan motors should be selected so
that nominal horsepower ratings are not exceeded.
Where a fixed bypassed air quantity is required, the
bypass damper may be provided with a minimum closure
device. However, some control range is sacrificed with
this method. If face and bypass control is not to be used,
a fixed bypass may be obtained by using a face and
bypass damper section with the face dampers removed.
The bypass of outside and return air mixtures
introduces high humidity air directly to the conditioned
space. When employing face and bypass control, it is
preferable to bypass return air only. This may be
accomplished as shown in Fig. 33.
Vibration Isolation
Four types of isolators are normally used to absorb
the vibrations produced by fan-coil equipment as well as
other types of rotating or reciprocating machinery. In
order of decreasing effectiveness and first cost, they are:
1. Steel coil springs
2. Double rubber-in-shear

Part 10. Air Handling Equipment | Chapter 2. Air Conditioning Apparatus

3. Single rubber-in-shear
4. Cork
Steel spring or rubber-in-shear isolators are available
for floor-mounted or suspended equipment. Ribbed
neoprene pads may be bonded to any of the isolators
noted above for floor-mounted units. These pads resist
horizontal movement, compensate for slight irregularities
in the floor surface, and protect floors from marring.
The proper bearing surface should be provided for
cork pad isolators as recommended by the isolator
manufacturer. Underloading does not permit the full
resiliency to be utilized, while overloading may result in
permanent deformation of the cork structure.
Similarly, if spring or rubber isolators are loaded past
the point of full compression, binding occurs and there is
no isolation.
Vibration isolation efficiency is the percentage of a
vibration of a given frequency absorbed by the isolator.
Thus, the vibration transmitted beyond the isolator is the
difference between 100% and the isolation efficiency.
Isolation efficiency is a function of the isolator
deflection when loaded and the disturbing frequency of
the machine isolated. For a fan or fan-coil unit the
disturbing frequency is the fan speed. Chart 7 shows the
relation between static deflection, disturbing frequency
and isolation efficiency for any case of vibration. In
addition, Chart 7 indicates the ranges of deflection for
which the various types of isolators are normally
obtainable.
As illustrated by Chart 7, for a given disturbing
frequency, isolator efficiency increases with deflection.
Since greater deflections are obtainable with springs than
with other types of isolators, springs provide the most
effective isolation over all frequencies. Cork is not an
effective isolation material for frequencies below about
3000 rpm.
A minimum vibration isolation efficiency of 85% is
usually satisfactory for ground floor or basement
applications in noncritical buildings. Upper floors may
require as high an efficiency as 93%, while critical upper
floors usually allow no less than 95%. When several

pieces of vibrating equipment are concentrated in one
room in a critical upper floor installation, the required
efficiency may approach 98.5%, and the transmission of
vibration to the floor should be considered in the building
design.
Whether floor-mounted or suspended, unit may be
mounted on a steel channel base which is then isolated.
Units may also he mounted on vibration isolators directly,
with no intermediate base. Manufacturers provide support
points or hanging brackets for fan-coil equipment, and
their recommendations regarding support points and
isolator loadings should be followed. Often, larger units of
a series or those including the most components require
a channel frame base for mounting. Mixing boxes and low
velocity filter boxes are usually mounted on their own
isolators, so as to prevent a cantilever effect.
If a unit is to be directly isolated with no base
employed, the deflection at each Support point should be
the same. If equal loading is assumed, individual isolators
may be overloaded to the point of binding or
underloaded, resulting in decreased isolation efficiency.
Point loadings for a given unit, coil and component are
usually available from the unit manufacturer.
Operating weights should be used in selecting
vibration isolators. This is particularly important where
water coils are used.
Example 4 – Vibration Isolator Selection
Given:
Fan – coil unit operation weight – 1640 lb equally
distributed at four points.
Fan speed - 800 rpm
Design isolation efficiency – 90 %
Find:
Required type and characteristics of isolator.
Solution:
1. From Chart 7 read the required isolator
deflection of 0.6 in., within the spring application
range.
2. Determine the individual isolator loading; 1640 =
410 lb.
4
3. Select a spring isolator with a maximum
characteristic of 410/0.6 or 683 lb per inch. If the
characteristic of the spring chosen is less, the
deflection is greater than 0.6 in., and the
isolation efficiency greater than 90% . However,
the spring must not be loaded above its
maximum.
Filters
Factory–fabricated filter sections for both high velocity
or low velocity filters are normally obtainable from the
manufacturer of a fan–coil unit. Either throw–away or

Part 10. Air Handling Equipment | Chapter 2. Air Conditioning Apparatus

cleanable filters can be used. For built – up apparatus
field – assembled filter frames are available.
If high velocity filters are to be used in a low velocity
filter section, the full area of air flow is not required.
Rather than fill up the entire section with high velocity
filters operating at a low velocity, bank–off pieces may be
installed, thus lowering the effective area. Blank-offs
should be located uniformly across the face of the filter
section instead of concentrated in one place.
Filters are discussed in detail in Chapter -4 of this
part.
INSTALLATION
Location
The economic and sound level considerations
pertaining to the location of air handling apparatus, as
discussed in Part 2, are applicable to fan-coil equipment.
Two of the most important factors in the location of air
conditioning equipment are the availability of outdoor air
and the ease of air return. Outdoor air may be brought to
a unit thru a wall, roof or central building chase. It is
preferable to locate outdoor air intakes so that they do not
face walls of spaces where noise would be objectionable.
Air may be returned thru a duct system or directly to the
equipment room.

Layout
A fan-coil unit may be of the vertical or horizontal type,
depending upon the direction of air flow entering the fan
cabinet. It may be floor-mounted or, in the case of a
horizontal unit, suspended from above. The choice of unit
style and mounting usually depends upon space
requirements and optimum duct layout. A support base
may be employed, if necessary, as discussed under
Vibration Isolation.
A practical location recognizes the need for effective
servicing of the unit. A minimum of 30 inches is
suggested to provide access between the unit and the
nearest wall. This facilitates servicing of steam traps, fan
bearings, damper motors and fan motor. In addition,
service space about the unit must be provided for filter
removal, coil removal, fan shaft removal, an(l the cleaning
of cleanable coils.
Suspended units should be accessible from above, if
possible. If frequent access is required and space
permits, a catwalk may be required.
An access plenum and door should be provided
between the filter section and coil section of a spray fancoil unit. This access permits periodic inspection and
cleaning of the sprays and drain pan.
A level unit is necessary to insure proper drainage
from coils and drain pan. Manufacturers of spring

Part 10. Air Handling Equipment | Chapter 2. Air Conditioning Apparatus

vibration isolators usually provide leveling devices in the
isolator to compensate for deflection differences.
Units located outdoors require suitable motors and the
protection of fan drive and shaft bearings, as well as
insulation as noted below.
For information pertaining to the design of air
distribution system components and piping at the unit,
refer to Parts 2 and 3.
Insulation
In a fan-coil unit the casing housing the fan section,
cooling coil section and components downstream of the
cooling coil are usually internally insulated. This insulation
is adequate for normal interior applications. The outdoor
air intake duct should be insulated and vapor sealed to
prevent condensation on the duct during cold weather.
If the intake is kept as short as possible, insulation costs
are minimized. Insulation and vapor sealing of the mixing
box may be required, depending on the quantity of
outdoor air introduced and on the winter design
temperature. Intakes for units circulating 100% outdoor
air should be insulated up to the preheater.
Units located outdoors should be completely covered
and caulked with weatherproofing material. If the outdoor
air temperature can fall below the dewpoint of the air
within the unit, the unit should be externally insulated,
vapor sealed and weather-proofed to prevent interior
condensation and to minimize heat losses. The insulation
on the top surfaces of the unit should be slightly crowned
so that water can run off.
CONTROL
If an air conditioning apparatus is to perform
satisfactorily under a partial load in the conditioned area,
a means of effecting a capacity reduction in proportion to
the instantaneous load is required. The three methods
most commonly employed for capacity control are air
bypass control, chilled water control and air volume
control.
With a drop in room load the sensible heat ratio
usually decreases since the room latent load remains
constant. This condition commonly occurs in areas where
a large proportion of sensible load such as solar heat
gain may be decreased with no influence on latent load,
as from people or infiltration.
In order to maintain design conditions at partial loads

and with decreased sensible heat ratios, the effective coil
surface temperature for a given coil must be lower than
that surface temperature consistent with full load design
conditions. This requirement is illustrated in Fig. 34 . The
relation between effective coil surface temperature and
percent of design room sensible heat depends on the
volume of outdoor air conditioned by the coil.
However, decreasing the flow of chilled water thru the
coil as a means of capacity control causes the effective
coil surface temperature to rise as the load decreases.
Therefore, room humidity also rises. For this reason it is
preferable to maintain the design flow of chilled water thru
the coil at all times.

Figure 35 a shows a typical cooling coil process at full
load for a given set of entering air and water conditions.
Figures 35 b, 35 c and 35 d depict at half load the three
methods of control cited and the influence in each case
on effective coil surface temperature. Air volume control
is similar in effect to air bypass control. However, the
bypassing of air around the coil permits a relatively
constant air delivery to be maintained.

Part 10. Air Handling Equipment | Chapter 2. Air Conditioning Apparatus

Part 10. Air Handling Equipment | Chapter 2. Air Conditioning Apparatus

accompanied by blowing out the coil with a portable
blower to remove residual water. An alternate method of
freeze protection is to circulate an inhibited antifreeze
solution thru the coil before final drainage.
Operating the chilled water pumped during the winter
is a costly solution to the problem of freezing. In addition,
it is not a certain method since a plugged tube could still
freeze.
The practice of using a properly inhibited alcohol
brine or antifreeze thruout the year for coil freeze-up
protection is becoming more common. Brines have been
developed and are now available, particularly for this
purpose. Refer to Part 4.

WASHER EQUIPMENT

As discussed in Part 1, applications with high latent
loads may require reheat control of room temperature.
Figure 36 indicates the amount of reheat required,
relative to room total heat, to maintain design relative
humidity at various sensible heat ratios.
COIL FREEZE-UP PROTECTION
The freezing of water in preheat, reheat and chilled
water coils may damage the coils and lead to costly
repairs. Freezing may occur not only in coils of units
operating during cold weather but also in the coils of units
not in operation.
Outdoor air at subfreezing temperatures often comes
in contact with heat transfer surfaces as a result of air
temperature stratification. Stratification is caused usually
by incomplete mixing of return air and outdoor air or by
an uneven temperature rise thin the preheat coil caused
by coil throttling. The complete mixing of air may be
promoted by the proper arrangement and design of the
ductwork. Uneven temperature rises thru preheat coils
and heating coil freeze-up may be prevented as outlined
in Chapter -1 of this part.
Coil freeze-up may also be caused by the direct
introduction of cold air thru an unprotected coil.
Circulation of outdoor air thru an interior fan-coil unit not in
operation can be induced by a stack effect, particularly if
the unit is on one of the lower floors of a tall building.
In addition to design precautions against stratification,
the following methods may be employed to protect a
water coil:
1. Remove the water from the coil during the winter.
2. Run the chilled water pump.
3. Decrease the freezing point of the coil water.
Removal of the water from the coil should be

The most commonly applied type of washer
equipment is the central station washer (Fig. 37),
designed for incorporation into a field-built apparatus.
Figure 38 is a cutaway view of the same type of washer
and indicates the direction of air flow.
This washer consists of a rectangular steel chamber,
closed at the top and sides and mounted on a shallow
watertight tank of steel or concrete. Inlet baffles located at
the air-entering end of the washer promote uniform air
velocities thru the washer and minimize the spraying back
of water into the entrance chamber as a result of air eddy
currents. At the air-leaving end of the washer, eliminators
are provided to remove entrained water droplets
Within the washer spray chamber two banks of
opposing spray nozzles provide finely divided droplets of
water uniformly distributed. After contacting the air, the
water is collected in the tank and is returned to the sprays
by a recirculating pump.
A central station washer may be designed for use as a

Part 10. Air Handling Equipment | Chapter 2. Air Conditioning Apparatus

humidifier or as a dehumidifier. The physical arrangement
is the same in either case. A dehumidifier is normally
shorter in airway length than a humidifier.
Washers may also be obtained in a unitary design. A
unitary spray washer, comparable in design and function
to a central station washer, is shown in Fig. 39. Other
types of unitary washers involve the wetting of a fibrous fill
or set of pads located in the air stream.
The particular washer shown in Fig. 39 operates at
high spray chamber air velocities and is. therefore,
smaller than a central station washer for a given air
volume. Figure 40 indicates the path of the air thru the
unit components. The unit includes an inlet air mixing
plenum, a vaneaxial fan, a diffuser section, a spray
section and a rotating eliminator.

Two to six banks of sprays condition the air and clean it of
dirt and other airborne particles. After contact with the air,
the water drains from the spray section to a central tank
from which it is recirculated.
APPLICATION
Air washers are primarily employed in industrial air
conditioning applications. The use of sprays permits
humidification, dehumidification or evaporative cooling,
as required. In addition, sprays enable a degree of
humidity control not possible with coils alone.
Washer equipment is effective in the removal of
certain types of odors and dirt from the air. In applications
where coils could become clogged with airborne solid
particles, washers require a minimum of maintenance.
This flexibility of function is obtained at a relatively low
installed cost of equipment per unit of air delivery. A large
air capacity is realized from equipment of low weight.

Part 10. Air Handling Equipment | Chapter 2. Air Conditioning Apparatus

fan noise levels and lower fan operating costs. Since
central station washers are usually fewer in number and
more centrally located than unitary washers, they require
less piping when used as dehumidifiers for a given
installation.
Central station washers may be obtained for air
deliveries of 2000 to 336,000 cfm. Unitary spray washers
are available in the delivery range of 7800 to 47,000 cfm.

This type of equipment is, however, open hydraulically
and thus presents problems in piping design and system
balancing. Refer to Part 3 for a discussion of washer
piping. Since the flow of air and water thin the apparatus
is parallel and since a gravity return of water is usually
employed in a dehumidifier application, pipe sizes tend to
be larger in an open system and the piping system and
insulation more expensive.
The spraying of water at high pressures such as are
required in washer equipment produces a noise level
high enough to be objectionable under some
circumstances. Sound treatment is not usually required
on equipment serving manufacturing areas or areas of
high ambient sound levels. For more critical applications
the need for sound absorption should be investigated
The unitary spray washer shown in Fig. 39 requires
considerably less space than a central station type and
requires no special apparatus room. It is more flexible in
meeting the necessities of plant layout change and is
more adaptable to zoning. The salvage value is high and
the operating weight low.
The central station washer installation results in lower

Humidifier
A spray humidifier provides evaporative cooling
thruout the year, as required, and heating during the
winter season, if necessary. It is particularly suitable to
applications where large quantities of sensible heat are to
be removed, and where comparatively high relative
humidities are to be uniformly maintained without the
need for controlling dry-bulb temperature above a
prescribed minimum. This type of washer equipment has
been used extensively in the conditioning of industrial
facilities engaged in the manufacture or processing of
hygroscopic materials. Such industries include textiles,
paper manufacturing, printing and tobacco processing.
A system of supplementary room atomizers is often
used in conjunction with a spray humidifier in order to
lower the first cost of the system. The psychrometrics of a
combination system are outlined in Par1.
Spray humidifiers require the recirculation of water
with no refrigeration. Recirculation occurs at the
apparatus in the case of the central station washer. With
the unitary washer, the recirculation of the water is
produced centrally.
Dehumidifier
A spray dehumidifier provides sensible cooling and
dehumidification during the summer season, evaporative
cooling during the rest of the year, and heating, if

Part 10. Air Handling Equipment | Chapter 2. Air Conditioning Apparatus

necessary, during the winter. It is used where lower
relative humidities are to be uniformly maintained and
where dry-bulb temperatures are to be controlled at a
comfortable level. A source of chilled water is required for
this application.
In a multiple central station system installation, the
recirculated water quantity remains constant for each
washer, and the chilled water is introduced in varying
quantities at the suction of the recirculating pump during
the dehumidifying season. See Part 3. The excess water
returning to the washer tank is either pumped back to a
central collection tank or, more commonly, drained from
the washer to the central tank by gravity. If a gravity
return is employed , a weir is required in the washer tank
to maintain the water level in the tank. Rate of return in a
pump-back application may be varied by a control valve
actuated by the washer tank water level. A return pump
should be sized to provide from l0 % to 20% more
capacity than is required. In either case, the amount of
chilled water admitted to the apparatus should be limited
to a maximum of 90% of the recirculated water quantity.
Various central station tank arrangements are shown
in Fig. 41. Figures 41a and 41b apply to gravity return
dehumidifiers. Figures 41c and 41d are typical of
pumped return dehumidifiers or evaporative cooling
applications.
Although unitary spray washers may be arranged in
the same manner as central station washers, they are
usually supplied directly with chilled water with no
recirculation at the unit. Spray density, therefore, varies
with load. Water return is by gravity to a central collection
tank.
During the months that refrigeration is not required,
the chilled water pump serving the central station spray
dehumidifier is idle.
Although efficient heat transfer is promoted by the
direct contact of air and spray water in a washer, the
parallel flow of air and water is less conducive to heat
transfer than the counter flow process possible with a
coil. Figure 42 illustrates a method of obtaining a counter
flow process with a two-stage spray dehumidifier. Flow is
parallel thru each individual stage. Such an arrangement
may permit a higher chilled water temperature or a
smaller water flow.

UNIT SELECTION
The selection of a washer includes the determination
of optimum washer size and dimensions and the
establishment of the recirculated spray water quantity
and pressure.
In the case of a dehumidifier, a study of the economic
effects of a washer selection on other components such
as piping or refrigeration equipment may be required.
Increasing the recirculated water quantity or decreasing
the washer face velocity by selecting a larger washer can
permit operation at higher chilled water temperatures or
at lower chilled water quantities.

Part 10. Air Handling Equipment | Chapter 2. Air Conditioning Apparatus

decreases as face velocity increases. Thus, for a required
air temperature rise, slightly more air is required at higher
washer face velocities. However, the effect is not
economically significant enough to justify lower washer
face velocities.
The unitary spray washer (Fig. 39) operates at
velocities up to 2600 fpm with efficient elimination of
entrained moisture. This type of washer is rated to handle
a nominal air quantity, and selections are made in tile
range of 75% to 105% of nominal.

Unit Size
The face area of a washer is determined by the
design air quantity and the recommended maximum face
velocity. Dehumidifiers are normally designed to operate
at velocities of 300 to 650 fpm. Humidifiers are usually
selected in the 300 to 750 fpm velocity range. Velocities
above or below these limits are not conducive to efficient
eliminator performance. Therefore, if a washer must be
oversized to provide for future capacity and if the
resulting face velocity is less than 300 fpm, a partial
blank-off of the face area is suggested to increase the
velocity until that time when full capacity is required.
Similarly, if volume control is used to maintain space
conditions ,the air velocity should not be allowed to drop
below 300 fpm.
For maximum economy and flexibility of control, it is
suggested that washers be selected at a face velocity as
near as possible to the recommended maximum.
With an approximate face area determined, several
washers of various heights and widths may be selected.
First cost of the washer is usually minimized if it is
selected as square as possible, with the height
approximately equal to the width. However, at washer
heights above a specified maximum the manufacturer
may stack eliminators, thus in effect creating two
washers. It is preferable economically to select the
washer with a height below this maximum, even if the
washer width then exceeds the height.
Washer saturation efficiency or contact factor

Spray Water
Washer saturation efficiency and contact factor are
determined by various spray characteristics in addition to
face velocity. These characteristics include the number of
spray banks and the spray water pressure. At a given
spray pressure, spray water quantity may be varied over
a relatively wide range with little change in contact factor
or saturation efficiency. This can be accomplished with
different combinations of spray nozzle orifice size and
number of nozzles.
Spray pressures usually lie in the range of 20 to 40
psig, with the higher pressures producing higher
saturation efficiencies. Dehumidifiers normally require
lower spray pressures than humidifiers.
At a given recirculated water quantity the fewer the
number of spray banks, the greater the saturation
efficiency since the spray pressure is greater. However,
in the design and rating of central station washers, tile
number of spray banks available is usually standardized
and limited.
Optimum dehumidifier efficiency is usually obtained at
a spray water quantity of approximately 5 gpm per
square foot and a pressure of 25 psig. The spay density
may vary from 3 to 11 gpm per square foot without an
appreciable effect on the performance, providing the 25
psig nozzle pressure is maintained.
Humidifier spay densities vary from 2.25 to 3.0 gpm
per square foot depending on the number and size of
nozzles used.
Evaporative cooling applications require only a
knowledge of size and saturation efficiency to complete
the selection. However, for a dehumidifier selection the
relation between leaving air wet bulb temperature and
recirculated water temperature after air contact should be
known. This information is necessary in order to calculate
the quantity of chilled water required at a given
temperature. Chart 8 illustrates such a rating.

Part 10. Air Handling Equipment | Chapter 2. Air Conditioning Apparatus

The unitary spray washer may be selected at various
water quantities. A selection of spray banks is therefore
available so that a range of contact factors may be
obtained. Dehumidifier ratings are based on the
apparatus dewpoint concept, as may be fan – coil unit
ratings. A typical dehumidifying performance for a given
unit size is shown in Chart 9.
Recirculating water pump heads for central station
washers usually range from 50 to 85 ft wg, provided the
pump is close to the washer. The pump head is primarily
determined by spray nozzle pressure.
Fouling factors used for selection of refrigeration
equipment used with washer equipment should be
minimum of .001. See Part 5.
Atmospheric Connections
No correction to wash ratings is necessary for
applications at altitudes above sea level. However, the
design air quantity should be determined as described in
Part 1, and the air side pressure drop of the washer
adjusted as outlined in Part 2.
The fan selection should be in a accordance with the
suggested procedure found in Chapter 1 of this part.
Motor selection at high altitudes is described in Part 8.

Part 10. Air Handling Equipment | Chapter 2. Air Conditioning Apparatus

ACCESSORIES
Flooding nozzles
For applications where solid airborne particles may
accumulate on eliminator blades, flooding nozzles may
be provided to continually flush the blades with
recirculated water. Flooding nozzles may also serve the
baffles at the entering face of a central station washer.
Baffle sprays, however, are usually necessary only in
applications with large quantities of air born list, such as
textile mills.
Eliminator flooding nozzles usually operate at spray
pressures of 3 to 10 psig for central station washers and
5 to 20 psig for unitary washers. With the central station
type of washer a spray water quantity of 4 gpm per row
per foot of washer width is suggested. One row is
generally required for each eliminator section. The
flooding of eliminator blades should be limited to those
blades of at least six bends.
Baffle spray nozzles may be designed for spray
pressures of 5 to 15 psig, and should be spaced to
provide effective baffle coverage at a spaced to provide
effective baffle coverage at a spray water quantity of 3 to
6 gpm per foot of washer width per pipe header. Headers
should be spaced 2 to 3 feet apart at the entering face.
Flooding nozzle water requirements may be furnished
by a separate recirculating pump or may be delivered by
the main recirculating pump. If the latter means is
chosen, the flooding nozzle water quantity should be
added to that of the main sprays and the total then used
to select the pump.
Water Cleaning devices
In order to insure proper spray nozzle operation and a
minimum of manual cleaning and maintenance, foreign
matter from the air stream and from eliminators and
baffles should be removed from the spray water.
Two types of cleaning devices are commonly
employed for this purpose: stationary screens and
automatic self-cleaning strainers. Self-cleaning strainers
are usually the rotating drum or endless belt type.
Stationary screens are located in the washer tank so
that spray water must pass thru them before being
recirculated. Cleaning the screens is a manual operation
and can be facilitated by using two screens in series,
supported by independent screen guides. The screen
openings should be smaller than the spray nozzle orifice
size. The washer tanks shown in Fig. 41a and 41c should
be equipped with stationary screens.
An endless belt self-cleaning strainer may be used
with or in place of the stationary screens and is suitable
mainly for applications where foreign matter particles are
of a relatively large size. It operates continuously,
collecting the particles on a belt and then flushing them

with recirculated or city water from the belt into a basket.
If recirculated water is used, the requirement should be
added to that of the main spray and flooding nozzles in
order to determine the required pump capacity. A blet
strainer can be located within the washer tank (Fig. 41b
and 41d). A belt strainer is shown in Fig 43.
The rotating drum strainer is installed in a central
storage and collection tank. It is a more efficient cleaning
device than the stationary screen or belt type strainers.
For this reason it is particularly suited for use with a
unitary spray washer system where all water is returned to
a central location and where the tubes of a water cooler
and various control valves must be protected from foreign
particle accumulations. Figure 44 illustrates a rotating
drum strainer.
With this method of cleaning water is filtered
continuously thru a perforated drum. Accumulations of
residue on the drum surface cause the water level to ruse
in order to seek more perforations. The variations in water
level control the periodic drum and to remove the waste
matter to a collecting basket.
Spray Water Heaters
Spray water heaters may be required in winter when
the mixing of outdoor and return air upstream of the
washer cannot be controlled to produce the washer
entering wet-bulb temperature required to maintain room
design conditions with an evaporative cooling process.
This condition may occur on very cold days or where
minimum outdoor air requirements are relatively high,

Part 10. Air Handling Equipment | Chapter 2. Air Conditioning Apparatus

The high velocity unitary washer utilizes a steam grid
humidifier for humidity control under the conditions
described above. Since steam is released directly to the
air, relatively little sensible heating is accomplished.

particularly with high room relative humidities and/or high
room sensible heat ratios.
After a shutdown, as over a winter weekend, it may
take some time to bring the room humidity up to design
conditions when operating on an evaporative cooling
cycles, even when the outdoor air quantity is reduced to
damper leakage. Therefore, the spray water heater is
used to add moisture to the air at approximately the same
dry-bulb temperature. The heater thus provides a
deviation from an adiabatic saturation process (Fig. 45).
Both steam ejector heaters and closed water heaters
are available for heating spray water in central station
washers. The steam ejector heater is a perforated steel
pipe, closed at one end and submerged in the washer
tank. Low pressure steam is admitted directly to the
washer tank water at a controlled rate. The closed water
heater is located on the discharge side of the
recirculating pump and is installed in parallel with the
main recirculating supply line. It is selected to heat a
minimum quantity of spray water and requires suitable
service and balancing valves. The closed water heater
produces less noise than the steam ejector heater and
enables recovery of steam condensate. However, the
closed water heater is more costly to purchase and
install.
A spray water heater may be sized on the basis of
requirements calculated at the particular conditions
encountered. Its capacity may also be calculated by
determining the steam quantity required to heat and
humidity the minimum outdoor air, or outdoor air damper
leakage, from outdoor to room design conditions. The
latter method provides sufficient capacity to maintain
room design conditions during the period following
equipment start-up.

Weirs
A weir is employed in a central station spray
dehumidifier tank to insure a minimum submergence of
the recirculating pump suction pipe and to maintain a
water seal under the eliminators. During the evaporative
cooling season there is no flow over the weir. During the
dehumidifying season, however, the flow is equal to the
chilled water admitted to the recirculating system plus the
moisture removed from the conditioned air.
Weir tanks may usually be obtained from the washer
manufacturer. If a concrete tank is to be used, the length
of the weir may be calculated from the Francis formula for
sharp-crested rectangular1.5weirs,
Q = 3.33 x L x H
The flow Q is expressed in cubic feet per second. The
length L and the head H are in feet. If the weir is to have
end contractions, the length should be reduced by 0.1 x
H for each end contraction, for formula use.
The sharp-crested concrete weir may be obtained by
bolting a steel angle to the flat crest.
A flow rate of 5 gpm per foot of weir length is common
for dehumidifier tanks.
Isolation
A central station washer requires no vibration
isolation. The supply air fan isolation requirements should
be investigated, however, with regard to the ambient
sound levels thruout the building. Unitary washers seldom
require vibration isolation for industrial applications but a
vibration analysis may be necessary for critical
installations. Isolation recommendations may be found
under Fan-Coil Equipment in this chapter.

Part 10. Air Handling Equipment | Chapter 2. Air Conditioning Apparatus

INSTALLATION
Location
The economic and sound level considerations
pertaining to the location of air handling apparatus, as
discussed in Part 2, are applicable to washer equipment.
Both central station and unitary spray apparatus may
be located indoors or outdoors, although central station
washers are most commonly located indoors, in an
apparatus room or in the conditioned space. Ii exposed
to the weather, a central station washer should operate
with no water level maintained in the tank, and the fan
motor, drive and bearing should be suitably chosen and
protected.
Central station equipment is floor-mounted while the
unitary washer may be either floor-mounted or suspended
from above (Fig. 46).
As with fan-coil equipment the availability of outdoor
air and the ease of air return to the apparatus is of
primary concern in selecting a washer location. Outdoor
air intakes should, if possible, be located and oriented so
that they do not face nearby residential areas or walls of
spaces where noise would be objectionable. Air may be
returned thru a duct system but, if it is returned directly to
the apparatus, the apparatus should be located so as to
receive return air from the area it serves.
Another important location consideration is the
availability of space. Particularly in industrial production
areas space may be difficult to acquire. Limited head
room and interferences such as electrical equipment,
conveyors or belt drives may also present problems.

In addition, the location and orientation of the washer
should be guided by the following considerations:
1. A spray dehumidifiers should be located so that the
gravity return of water to the central tank is possible. If
this condition cannot be met, a pumped return should
be employed.
2. Adequate building openings and passages should be
available for the admittance of large equipment. If this
consideration is overlooked, special openings in the
building may later be required.
3. Locating a spray dehumidifier below the refrigeration
equipment or pumping return water to a lower
elevation may lead to problems of siphoning or
overflow at shutdown. If such an arrangement is
necessary, consideration should be given to the
checking of water back-flow tendencies and the
breaking of a siphon.
4. The ability of a roof, floor or combination of structural
members to withstand the operating weight of a
washer should be investigated.
5. A washer should be located and oriented so that the
simplest possible duct layout results.
6. Appearance should be considered. For example, a
washer mounted on a flat roof may be less noticeable
if located some distance from the building perimeter.
Layout
Figure 46 illustrates several layout alternatives for a
high velocity unitary washer. A typical central station
washer apparatus room layout resembles that shown for
coil equipment in Part 2.

Part 10. Air Handling Equipment | Chapter 2. Air Conditioning Apparatus

Central station washers should be provided with inlet
plenums of adequate airway depth to promote outdoor
and return air mixing and to minimize air eddy currents at
the inlet face of the washer. The plenum chamber on the
leaving air side of the washer should be large enough to
provide unrestricted air flow to the fan at uniform
eliminator velocities. Plenums downstream of the washer
should also permit easy cleaning after removal of the
eliminator blades for both central station and unitary
apparatus.
Sufficient space should be provided around the
washer for maintenance access, particularly on the side
where access doors and piping connections are located.
Minimum clearance should be provided on the far side for
cleaning, painting and for the application of insulation if
required. If suspended well above the floor, unitary
washers may require catwalks.
Access doors should be installed between central
station apparatus components as required for proper
maintenance and service.
A mounting base at least two inches high should he
provided for central station equipment. A base provide a
level and uniform bearing surface for the tank, prevents
damage to the tank or to the insulation under the tank
from water seepage, and increases the tank water level
available for priming the recirculating pump.
If a concrete tank is designed for a central station
washer, it should be of reinforced construction and
provided with pipe sleeves, baffle and eliminator supports
and anchor bolts, as required. In addition the plenum at
the leaving air side of the washer should be provided with
a curb at least four inches high.
The recirculating water pump may be located in the
air stream entering the washer or outside of the washer
casing.
Marine lights should be provide within the washer and
between components of a central station washer.
In addition to the washer piping details shown in Part
3, the following suggestions apply:
1. Floor drains should be provided in the entering and
leaving air plenums, near the recirculating pump,
near the outdoor air intake and as required for the
cleaning of filters or other components. Usually the
drain in the leaving air plenum requires a deep seal
trap.
2. If a pumped return is used for a spray dehumidifier,

a continuously running 1/2 inch bleed line from the
return pump discharge to the tank prevents
overheating of the water in the pump when the
return control valve is closed.
3. Because the water in a washer tank is relatively
shallow, a well designed vortex breaker on the
pump suction pipe is required.
Insulation
The top and sides of a spray dehumidifier should be
insulated as required to prevent condensation on the
apparatus and to minimize heat transfer. A central station
humidifier should be similarly insulated if the dewpoint for
the return air is higher than the spray water temperature,
such as occurs when supplemental atomizer systems are
employed.
The high velocity unitary washer described previously
should be completely insulated, regardless of the
application.
Vapor harriers must be applied to the high dewpoint
side of the insulation to prevent condensation forming on
the metal surface of the unit.
A thickness of cork insulation may be required
beneath the washer tank. If used, the cork layer should
be coated on both sides with sealing compound and
positioned on the tank pad before the unit is installed.
Washers located outdoors should be insulated, vapor
sealed and weatherproofed. The insulation on the top
surfaces should be slightly crowned so that water will run
off.
Water and steam riser insulation in industrial
applications is sometimes subject to damage from trucks
and material handling equipment. If such is the case, a
sheet metal shield around the insulation is suggested,
from the floor to a height of several feet.
CONTROL
The function of controls is to produce a balance
between the air conditioning load and the apparatus
capacity in order to maintain room design conditions.
Apparatus control may be accomplished in one or a
combination of two ways:
1. Varying the supply air volume at a given air
condition.
2. Varying the air condition with no change in volume.

Part 10. Air Handling Equipment | Chapter 2. Air Conditioning Apparatus

The condition of the air may be altered by such
methods as spray water temperature variation, air
reheating, spray water heating, washer bypass, spray
throttling, variation of the outdoor and return air mixture
proportions, and air humidification, as with a steam grid
humidifier or an atomizer system.
A central station air washer operating on a
dehumidifying cycle normally utilizes spray water
temperature variation, air volume reduction and air
reheating. See Fig. 47 for the simplified control diagram.
A dry-bulb thermostat, located on the leaving air side of
the washer and set to maintain the leaving dry-bulb
condition necessary to achieve the required leaving air
dewpoint, controls the chilled water valve supplying the
recirculating water system. The spray water temperature
is thus controlled.
Dewpoint control is practical because the difference
between dewpoint and leaving air dry-bulb temperature is
small as a result of the high contact factor of the
dehumidifier.
Air volume and reheat control is obtained by a
thermostat and humidistat together controlling a reheat
coil bypass volume damper and a reheater steam valve in
sequence. Closing the volume damper reduces the
supply air volume to a predetermined fraction of full load
air delivery, usually from 60% to 85%, depending on

the fan characteristic and the allowable maximum air
pressure drop thru the heating coil. A further reduction in
room load causes the reheater valve to begin opening.
Figure 48a illustrates a typical spray dehumidifying
process at full load. Figure 48b shows the temperature
relations at half load. The entering spray water
temperature has been increased to maintain a relatively
constant apparatus dewpoint.
Chilled water supply to the central station dehumidifier
recirculating system can be controlled by a two-way
throttling valve or a three-way diverting valve. Use of a
two-way valve on a multiple washer system may
necessitate a pressure bypass line anti valve at the
central collection tank in order to minimize line pressure
fluctuations.
Dewpoint control of a central station air washer
operating on an evaporative cooling cycle is achieved by
controlling the outdoor and return air mixture condition
and by operating the spray water heater, if necessary.
Zone control may be identical with that control employed
when on the dehumidifying cycle.
A measure of humidity control may be obtained during
a period of refrigeration shutdown by cycling the
recirculating pump and/or the baffle or eliminator spray
pump, if any.
In a high velocity unitary washer application, room

Part 10. Air Handling Equipment | Chapter 2. Air Conditioning Apparatus

conditions are controlled directly by a combination of
spray throttling, air reheating and, if necessary,
humidification. Figure 49 is a control diagram for units
operating with dehumidification control for an all air
system. During the dehumidifying season the outdoor air
dampers are in a minimum position; the reheat valve is
con trolled by the room thermostat alone; the spray
throttling valve is controlled thin the pressure selector,
and the humidifier is controlled by the humidistat,
providing the thermostat is satisfied. When operating on
the evaporative cooling cycle, the outdoor and return air
dampers are controlled by the room thermostat: the
reheat valve is controlled thru the pressure selector; the
spray throttling is controlled by the humidistat, and the
humidifier is controlled as in the dehurnidifying cycle.

Since spray throttling is always utilized with this
system, a pressure bypass line and valve are usually
required at the central water collection tank to minimize
fluctuation in line water pressures.
General Control Considerations
For industrial applications, particularly in the case of
process air conditioning, room controls are often mounted
within a cabinet provided with a small fan. The circulation
of room air thru the cabinet provides a constant and
positive sampling by the control sensing devices.

Part 10. Air Handling Equipment | Chapter 2. Air Conditioning Apparatus

If large open spaces are to be conditioned, the area
served by each set of room controls should be limited in
order to maintain an adequate control response.
Maximum areas of 10,000 square feet for a temperature
zone and 8000 square feet for a humidity zone are
suggested.

Control accuracy and response are for also affected by
the air circulation in the conditioned space. A maximum
of ten minutes for a complete change of air is
suggested, and four to eight minutes is preferred.

Part 10. Air Handling Equipment | Chapter 3. Unitary Equipment

CHAPTER 3. UNITARY EQUIPMENT

A unitary air conditioning unit, sometimes referred to
as packaged equipment, consists of one or more factoryfabricated assemblies designed to provide the functions
of air moving, air cleaning, cooling and dehumidification.
The functions of heating and humidifying are also usually
possible with such equipment. Heat pump versions are
available for most types of apparatus.
Unitary equipment includes a direct expansion or
chilled water cooling coil and a compressor-Condenser
combination or water chiller in addition to fans, auxiliaries
and internal wiring and piping. If more than one assembly
is required, the separate assemblies are designed for use
with each other, and combined equipment ratings are
based on matched assemblies of equal or differing
nominal capacities.
The design of unitary equipment is often styled for
installation within the conditioned space.
It is the purpose of this chapter to guide the engineer
in the practical application and selection of unitary
equipment.

TYPES OF EQUIPMENT
Unitary equipment may be classified as either a selfcontained or a split system. A self-contained unit houses
all components in a single assembly. Split system
equipment incorporates the following assemblies
1. A coil and compressor combined with a remote
condenser.
2. A coil combined with a remote condensing unit.
3. A coil combined with a remote water chiller.
A self-contained unit is illustrated in Fig. 50. The selfcontained concept is further described in Fig. 51. Figure
52 shows an air-cooled condensing unit — one
component of a two-component split system as
described in Item 2 above.
The use of matched components differentiates unitary
equipment from the fan-coil equipment discussed in
Chapter 2 of this part. Unitary equipment thus affords
less flexibility of arrangement and less choice of cooling

Part 10. Air Handling Equipment | Chapter 3. Unitary Equipment

specifically required. As mentioned previously some
additional application flexibility may be obtained by
employing split system equipment. Sensible heat ratios
as high as 0.95 are attainable. Such equipment also
affords greater choice of location and mounting method.
Self-contained equipment is commonly available in
capacities up to 60 tons, while up to 75 tons may be
obtained with a split system. The trend has recently been
toward larger packaged equipment .
Water-cooled, air-cooled or evaporative condensing
may be utilized with unitary apparatus.
coil surface. Also, face and bypass control is usually
unavailable in packaged equipment.
Split system apparatus provides in packaged form a
measure of flexibility not usually obtainable with selfcontained equipment.

APPLICATION
The use of unitary equipment should be considered
for applications where the following advantages are of
primary importance:
1. Low first cost of equipment and installation.
2. Immediate air conditioning benefits and prompt
delivery.
3. Ease of installation or removal, if necessary, with a
minimum of disturbance.
4. The ability to provide air conditioning in increments
without cost penalty.
5. Economical operation during periods of nonuniform
loading.
6. High salvage value and longer warrantee periods.
7. Simplified field engineering.
8. Factory assembly of balanced and tested
components.
Packaged equipment is particularly well suited to
applications requiring summer cooling only, and is readily
used in conjunction with existing or separate heating
facilities of sufficient capacity.
Such equipment may effectively augment central
station apparatus by serving relatively small areas with
special design requirements. Typical applications of this
nature are laboratories and dining areas.
Applications completely conditioned with unitary
equipment include existing office buildings and hotels,
motels, shopping center tenant areas, department stores,
industrial facilities and residences.
Equipment components are usually matched to
provide 300 to 500 cfm per ton of air conditioning at
sensible heat ratios of 0.65 to 0.85, in the case of selfcontained equipment. Therefore, packaged equipment is
most economically applied where these values are

STANDARDS AND CODES
Applicable provisions of the American Standard
Safety Code B9.1, ARI Standard 210 and Underwriters’
Laboratories Standards govern the testing, rating and
construction of unitary air conditioning equipment.
The application and installation of such equipment
should conform to pertinent government agency
regulations and to all codes and laws prevailing at the job
site.

UNIT SELECTION
SELECTION RATINGS
Unit size is usually determined by the required cooling
capacity and air quantity, adjusted to suit the sensible
heat ratio. Cooling ratings present total and sensible heat
capacities, based on air quantity, evaporator entering air
wet-bulb temperature and, in the case of water-cooled
equipment, condensing temperature. A typical cooling
rating table is illustrated in Fig. 53. Although tabular
cooling ratings are most common, some manufacturers
present graphical data in place of, or in addition to,
tabular ratings.
Cooling ratings (Fig. 53) may be expanded to apply to
more than one evaporator entering air dry-bulb
temperature. If they are not expanded, deviation
corrections are usually suggested. Cooling rating also
may indicate grand sensible heat factor rather than total
sensible heat capacity.
For air-cooled condensing or evaporative condensing,
selection ratings are normally based upon condenser
entering air dry-bulb temperature or wet-bulb temperature
respectively, instead of condensing temperature.
Self-contained equipment is rated as a system with no
individual component ratings. A split system apparatus is
usually rated both as an individual item of equipment and
in combination with its intended components. For
example, an air-cooled condensing unit may be rated in
terms of cooling capacity available from the package
when the condensing unit is used with a particular
fan-coil unit at different evaporator wet-bulb and outdoor
dry-bulb temperatures.

Part 10. Air Handling Equipment | Chapter 3. Unitary Equipment

ECONOMICS
The cooling ratings described above are based upon
the capacities of components in balance with each other.
It is therefore usually unnecessary to determine
component balance capacities when selecting
packagedequipment. However, if equipment is to be
economically selected for use with unmatched
components, the determination of the component balance
points and the subsequent selection analysis should be
pursued as described in Part 7.
The economical balance of component capacities may
be upset if the grand sensible heat factor required for an
application differs considerably from that characteristic of
the package. For example, with relatively high sensible
heat ratios the desired air quantity per ton of capacity is
also high, relative to that available from standard
packaged equipment. With self-contained equipment,
therefore, it may be necessary to provide oversized
refrigeration components, in order to acquire the air
appropriate air delivery. In this case an economical
balance can be restored by designing for lower relative
humidities, thus permitting a greater air temperature rise
and a smaller air quantity. An alternate solution,
applicable within limits, is to vary the evaporator air
quantity. Split system equipment may be mixed as to
nominal component capacities to produce an economical
balance.
Economy of equipment selection may also be promoted by the following methods:
1. Select equipment to be fully loaded, taking
advantage of room temperature swing, storage
effects and reduced safety factors.
2. Avoid the arrangement of unit by zones where
selections must be made for peak loads.
Diversity benefits may be realized if more than
one exposure is served by a unit.

3.
4.

Consider operation at relatively high condensing
temperatures with possible savings as explained
in Part 7.
Introduce the least outdoor air possible at peak
apparatus load.

ATMOSPHERIC CORRECTIONS
Unitary equipment ratings are based on air at
standard atmospheric conditions of 70 F and 29.92 in. Hg
barometric pressure. For applications deviating
significantly from this standard such as at altitudes
exceeding 2500 feet, ratings should be adjusted for the
difference in air density. The several corrections involved
have been described elsewhere and may be summarized
as follows:
1. Load calculations should be modified as described
in Part 1.
2. Air side pressure drop should be adjusted in
proportion to the air density ratio as outlined in Part
2.
3. If the apparatus includes an evaporator, the unit
capacity should be corrected by entering the rating
tables at a supply air quantity equivalent to standard
atmospheric conditions. This procedure is similar to
that described for fan-coil units in Chapter 2 of this
part.
4. Fan speed and brake horsepower should be
adjusted as detailed in Chapter 1 of this part.
The decrease in performance of air-cooled
condensers at high altitudes and/or air temperatures,
although in itself significant, produces only a slight
deviation in the rating of combined components. This
deviation may amount to from one to three percent at an
altitude of 5000 feet.

Part 10. Air Handling Equipment | Chapter 3. Unitary Equipment

COIL FREEZE-UP PROTECTION
The freezing of hot water coils located downstream of
the cooling coil may occur during the cooling season.
This is particularly possible in packaged fan-coil
equipment because of the proximity of the heating and
cooling coils and because of the relatively low settings
usually employed on the low pressure compressor cutoff
switch to prevent excessive cycling. At lower evaporator
wet-bulb temperatures, equipment components balance
at diminished suction temperatures. Hot water coil freezeup may occur under these conditions.
Although coil freeze-up may be prevented by the
draining of the hot water coil or by the use of a properly
inhibited antifreeze solution as described in Chapter 2 of
this part, the use of a protective thermostat to cycle the
compressor is suggested instead in the interest of
economy. The thermostat should be mounted outside of
the air stream with the bulb at the entering face of the hot
water coil. An air temperature of approximately 35 F is
suggested as a compressor shutoff point.
Coil freeze-up may sometimes be traced to reduced
air quantities resulting from dirty filters in the apparatus.

INSTALLATION
LOCATION
Unitary air conditioning equipment may usually be
located either outdoors or indoors. Possible specific
locations include basements, crawl spaces, attics,
garages, roofs, and on the ground as well as within the
conditioned space or in an equipment room. Such
equipment may be mounted on the floor, suspended from
the ceiling or installed in a wall opening, transom or
window. Since packaged apparatus is sometimes
designed for a specific location such as on a roof or
under a window, the manufacturer’s literature should be
studied for location recommendations. The suggestions in
Part 2, Part 7, and Chapter 2 of this part regarding
equipment location are applicable to packaged
equipment.
Although weatherproofing kits are available for most
self-contained equipment, an indoor location is preferred.
However, self-contained equipment designed specifically
for outdoor use is available. With any equipment featuring
Outdoor compressors, crankcase heaters should be
employed to prevent the migration of refrigerant to the
compressor and the damage which may result. For
outdoor or indoor locations insulation considerations
apply as noted in Chapter 2 of this part.
LAYOUT
The references noted above should be consulted for
suggestions dealing with equipment layout.

Unitary apparatus is available in both horizontal and
vertical arrangements and it is usually designed for use
with or without duct systems. However, distribution
systems should be simple in design and limited in extent.
Often, unitary equipment may conveniently use the
same air distribution system as an existing heating
system. This is particularly true in residential applications.
In such an installation appropriate shutoff or diverting
dampers may be required if the heating and cooling
functions are in parallel. Existing duct sizes should be
checked for adequacy in handling the dehumidified air
quantity.
For roof-top installations the roof must be of adequate
strength, and the equipment weight should be evenly
distributed on the support members. If any doubt exists
as to the adequacy of support, a structural engineer
should be consulted. Appropriate framing around roof
openings, flashing, counter flashing and pitch pockets
should be provided.
External vibration isolation of packaged equipment is
seldom required because the individual Components are
usually isolated within the cabinet.
However, for critical installations and light building
construction, vibration isolation should be considered for
unitary equipment as for any other type of equipment.
Vibration isolation is discussed in Chapter 2 of this part.
Isolation recommendations may also be solicited from the
manufacturers of vibration equipment.
Layout and location of unitary equipment are
influenced by the availability of service facilities such as
gas, city water and electrical power.

CONTROL
The reduction of packaged equipment capacity at
partial loads is usually effected by cycling the
compressor or compressors in accordance with the
setting of a room dry-bulb thermostat. decreasing
compressor capacity by the unloading of cylinders is
another widely employed method of control.
Unit fans may be created continuously or cycled with
the compressor. Continuous operation of fans provides
continuous air circulation. However, alternately
condensing moisture from the air and reevaporating coil
moisture produces fluctuations in room humidity
conditions. Cycling of fans requires the use of a room
thermostat rather than a return air thermostat.
The desirability of controlling equipment capacity and
the unavailability of face and bypass control may affect
adversely the equipment capacity for latent heat removal.
For example, with a single compressor serving a single
evaporator coil, cylinder Unloading causes an increase in
the grand sensible heat ratio and thus a relative decrease
in the latent heat capacity. This occurs at a time when the

Part 10. Air Handling Equipment | Chapter 3. Unitary Equipment

opposite effect is usually desired.
This effect may be overcome by the use of multiple
compressors with multiple coils or coil circuits operating
on signals from a two-step thermostat. This permits an
improvement in latent heat removal at partial loads. Coils
may be of equal or unequal capacities. in either case,
some additional latent capacity is obtained by the
decrease in air quantity over the operative coil as the
inoperative coil dries. With unequal coils, where the larger
is the first to be removed from operation, additional latent
capacity is also obtained thru the lower sensible heat
ratio of the smaller coil and the prolonged ‘‘on’’ period of
the operating cycle.

Multiple compressors are usually obtainable only on
equipment of ten tons capacity, or greater.
Loss of latent capacity at partial loads and the
resulting fluctuations in room conditions are intensified by
the oversizing of equipment. Fully loaded equipment
provides the best insurance of maintaining reasonable
humidity conditions at partial loads. Unit air deliveries
may also be varied from nominal to obtain the most
desirable full load sensible heat ratio, and therefore the
best possible latent capacity at partial load.
Under special conditions where accurate control of
temperature and humidity is required, packaged
equipment may be easily adapted for control by reheat or
humidification.
The necessity and means of condensing pressure
control is discussed for various condensing methods in
Part 7.

Part 10. Air Handling Equipment | Chapter 4. Accessory Equipment

CHAPTER 4. ACCESSORY EQUIPMENT

This chapter presents practical information to guide
the engineer in the application and layout of air cleaning
and heating devices, as used in conjunction with air
conditioning systems.

AIR CLEANERS
The control of air purity consists of reducing or
eliminating unwanted particulate or gaseous matter from
the air supplied to a space. This is a function of the air
conditioning system. However, normal applications are
concerned with particulate matter only.
Effectively applied air cleaners can materially reduce
operating expenses and increase productivity. Specific
benefits include:
1. The reduction of building cleaning costs — an item
otherwise accounting for as much as forty percent
of total operating expenses.
2. The reduction of employee absenteeism — a result
of the removal of bacteria, viruses and allergens
from the air.
3. An increase in employee efficiency.
4. An increase in product quality.
5. An increase in the life of machinery or equipment.
CONTAMINANTS
Air is contaminated in varying degrees by soil, organic
matter, spores, viruses, bacteria and allergens, as well as
aerosols such as smokes, dusts, fumes and mists. These
contaminants may be introduced into the air from
outdoors, or they may be returned to the air conditioning
apparatus from within the space. The ease and efficiency
with which they may be removed depends on the size,
shape, specific gravity, concentration and surface
characteristics of the particle.
Contaminant characteristics vary widely. Particle
diameters range from molecular size up to 5000 microns*.
Concentrations as high as 400 grains per 1000 cubic feet
may be encountered. However, air conditioning
applications usually involve the removal of particles no
smaller than 0.1 micron in diameter and as large as 200
microns. Normal concentrations seldom exceed 4 grains
per 1000 cubic feet. The specific characteristics of the
particles to be removed are determined by the
(*One inch equals 25,400 microns.)

application. Thus, air purity control is a relative concept.
The sizes of common contaminant particles are shown
in Chart 10. Typical outdoor air dust concentrations for
various localities are noted in Table 9. Concentrations
may increase during the heating season, especially in
residential areas.
Particles of an oily nature with irregular surfaces
electrostatically tend to agglomerate more readily. The
settling and adherence of contaminants is, therefore,
affected by other characteristics in addition to size and
concentration.

PERFORMANCE CRITERIA
Atmospheric air cleaners (referred to as air filters) are
rated in terms of efficiency (arrestance), resistance to air
flow, and dust capacity. The three most critical
performance factors are the following:
1. The variation of filter resistance with air flow.
2. The variation of filter resistance with dust load at
design air flow.
3. The effect of dust loads at design air flow on filter
efficiency.
Performance data for a typical unit filter are illustrated
in Fig. 54 Filter resistance increases with air flow (face
velocity) or with dust load at. design air flow. The
efficiency of a particular filter varies not only with dust
load hut also with the characteristics of the contaminating
particles. For this reason, Fig. 5-4 specifies the test
procedure used to rate the filter.
The capacity of a filter is a measure of its usable life
prior to disposal, renewing or cleaning.

Part 10. Air Handling Equipment | Chapter 4. Accessory Equipment

STANDARDS AND CODES
Air filter manufacture and installation should conform
to the recommendation in Pamphlet. 90A of the National
Board of Fire Underwriters and to all codes and laws
applying at the job site.
The efficiency and capacity of air filters are
determined by several standardized test methods,
differing primarily in the test aerosol used and the method
of measuring the amount of dust passed by the filter. The
three most common test procedures are these:
1. The weight method, with test aerosols as
specified by the Air Filter Institute Code and a
modification of the former ASHVE Code.
2. The dust spot method, with procedures as
standardized by the Air Filter Institute and the
National Bureau of Standards.
3. The D.O.P.* test, a particle count method utilizing
a chemical smoke aerosol.
* Di-Octyl-Phthalate

Part 10. Air Handling Equipment | Chapter 4. Accessory Equipment

These three methods (lifer in application and the
results are difficult to convert to common terms. In
comparing the performance of various filters it is therefore
imperative that the test used to obtain the published data
l)e noted in each case.
The weight method expresses filter efficiency in terms
of the particle weight removed, relative to the weight
introduced to the air stream. It is particularly useful in
evaluating the performance of mechanical filters of
average efficiency. However, this test overstates filter
effectiveness in removing small particles of light weight.
The dust spot test rates filters in terms of the relative
opacity of stains on filter paper thin which the air is
passed. The optical density of the spots are measured
photometrically. This type of test is useful primarily in
evaluating air cleaning devices of high efficiency, such as
electronic air cleaners. In addition it provides a measure
of filter efficiency in removing the sort of (lust most likely
to cause discoloration of walls and ceilings. Test results
are, however, sometimes inconsistent and are difficult to
interpret.
The D.O.P. test relates filter performance to the lightscattering tendency of smoke particles approximately 0.3
microns in diameter. Measurements are made photo
electrically. The test is used primarily to determine the
ability of filters of very high efficiency to remove specific
particles, such as pollen. It requires carefully controlled
laboratory conditions and expensive equipment. It cannot
be used to determine filter capacity.
TYPES OF AIR CLEANERS
Viscous Impingement
Filters of the viscous impingement type utilize a
filtering medium relatively coarse in texture and
constructed of fiber, screen, wire mesh, metal stampings
or plates. The medium is coated with a viscous substance
such as oil or grease. As the many small air streams
abruptly change direction thin the filter, contaminating
particles are thrown against the medium where they
adhere. Efficiencies of 65% to 80%, based on the weight
method of testing, are achieved in the case of cleanable
media.
This type of filter is available in a throwaway style or
may be obtained with a replaceable medium, a manually
cleanable medium, or an automatically renewed medium.
Media designed for filtering velocities of
approxImately 300 feet per minute usually increase in
density in the direction of the air flow. Thus, the larger
particles are the first removed, prolonging filter life. This
progressive density is illustrated in Fig. 55. High velocity
filters operating at approximately 500 feet per minute are
normally nondirectional and of uniform density. Figure 56
shows a cleanable viscous impingement panel filter.

Automatic viscous impingement filters may be of the

replaceable media or renewable media type. The former
consists of a moving filter roll. The latter is constructed of
overlapping filter panels attached to a moving chain and
moving thru an oil bath. The self-cleaning filter is shown in
Fig. 57. In either case the filter curtain may he actuated
by a timing mechanism or a pressure sensing device.
Automatic filters present a relatively constant resistance
to air flow, while panel filter resistance varies
considerably as the dust load increases. Automatic filter
efficiencies vary from 80 % to 90 % , based on the weight
method.
Dry Media
Dry filters consist usually of a permanent frame and a
dry replaceable medium of cellulose, asbestos or glass
fibers, specially treated paper, cotton batting , wool felt or
synthetic material. The air passages thru the medium are

Part 10. Air Handling Equipment | Chapter 4. Accessory Equipment

smaller than those of the viscous impingement type filter,
and therefore lower air velocities are necessary to avoid
excessive resistance . In order to obtain a large surface
area relative to cross – sectional area, the medium is
usually pleated in accordion form.
Figure 58 shows a medium efficiency dry filter with an
area ratio of 7 : 1 Such a filter is capable of a wide

efficiency range varying from 84 % to 95 % based on the
AFI weight test, depending on the medium used.
Figures 59 and 60 illustrate vary high efficiency filters
with area ratios of from 25:1 to 50:1. This type of filter
may be obtained with and efficiency as high as 99.97 % ,
D.O.P. test method. D.O.P. efficiencies above 90 % are
usual.

Part 10. Air Handling Equipment | Chapter 4. Accessory Equipment

Dry filters are available in automatic construction,
normally utilizing a moving roll of disposable medium
( Fig. 61 ). Movement may be controlled by a differential
pressure sensing device. Thus , operating air resistance
is maintained relatively constant.
The efficiency of a dry filter depends on the size and
spacing of the fibers in the medium used. Media with the
smallest, most densely distributed fibers provide the
highest efficiencies. High efficiencies, however, are
usually associated with high resistance, short life and low
dust holding capacity.
Electronic
Electronic air cleaners, often referred to as
precipitators, are of two varieties: the ionizing type and
the charged media type. They are illustrated respectively
in Fig. 62 and 63.
The ionizing type of electronic air cleaner ionizes
contaminating particles by passing the air thru an electric
field of approximately 12,000 volts potential. The particles
are then collected on charged plates which are usually
coated with an adhesive to prevent re-entrainment of the
particles. Efficiencies of 85% to 90%, based on the dust
spot test, are achieved. The collecting stage operates at
approximately 6000 volts. The high voltages are obtained

from rectifiers supplied with 110/120 volt single phase
electrical service. Power consumption varies from 12 to
15 watts per 1000 cubic feet per minute, with an
additional 40 watts required o energize the rectifier tube
heaters.
The charged media electronic air cleaner consists of a
panel filter with an electrostatically charged dry medium.

Part 10. Air Handling Equipment | Chapter 4. Accessory Equipment

It therefore combines the principles of electronic
precipitation and dry mechanical filtration. The produced
efficiency averages about 60% by the dust spot test
method. Approximately 12,000 to 1 3,000 volts are
required to charge the dielectric medium. Power
requirements are about 8 watts per 1000 cubic feet per
minute.
Although the ionizing type of electronic cleaner may
be obtained in replaceable cell construction, the
precipitator shown in Fig. 62 is automatically selfcleaning. Moving collector plates are cleaned and reoiled
by the same method as employed with the automatic
viscous impingement filter (Fig. 57). Semiautomatic
cleaning is also available, utilizing nozzles for water
cleaning and reoiling.
The resistance to air flow imposed by an ionizing type
electronic cleaner is very small. For this reason the unit
may feature screens or perforated plates at the air
entering and/or leaving side to promote uniform air flow
thru the precipitator.
APPLICATION
The choice of a particular type of air filter for a given
application involves the following steps:
I. A determination of the size, concentration and
characteristics of contaminants present in both
the outdoor air and return air.
2. A decision regarding the size of particles to be
remove and the efficiency required for removal.
3. The selection of the filter which will provide most
economically the desired efficiency under the
prevailing conditions of labor cost, power costs
and annual hours of operation.
Air contamination may be appraised by costly
laboratory analysis or by an estimation based on past
experience and general data. The latter method is
preferable in all except highly specialized applications.
Chart 10 and Table 9 may be used with judgment to
determine air contamination. Additional data of local
interest may be obtained from the city Bureau of Health or
smoke control agency.
The determination of which contaminants are to be
removed, and to what degree, should be based on the
requirements of the processes, equipment, material or
occupants within the conditioned space. For example, a
greater filtering efficiency would be required for an
electronics laboratory than for a bowling alley. However,
for any application certain contaminants should be
removed. These contaminants include abrasive (lusts,
lint, pollen, concentrations of toxic fumes, ii present, and
carbon, if in appreciable quantities.
Chart 10 also indicates the approximate normal
ranges of application of the various filter types based

upon particle size only.
Viscous impingement filters efficiently remove
contaminating particles larger than 10 microns in
diameter, particularly if the particles are oily. The coarse
media use (1 are well suited to large particle sizes anti
concentrations. The capacity and life of such filters are
great and therefore maintenance is relatively inexpensive.
High velocity unit filters (500 feet per minute) are not
suitable to heavy lint applications since the media density
is not progressive.
Dry media filters are more efficient than viscous
impingement filters in removing particles in the submicron
range. However, capacities are smaller with the finer
media used. Filters of average to medium efficiency are
useful in the collection of lint. High efficiency filters are
intended primarily for removing 1)articles of small size and
concentration. Dry filter life is relatively short, anti
therefore, maintenance costs are usually higher than for
impingement filters.
Automatic dry filters of the roll medium type are
seldom used to remove atmospheric dust. They are well
suited to the removal of lint as found in textile mills or dry
cleaning establishments, and may be used for the
removal of ink mist in the printing industry.
High efficiency dry filters (Fig. 59 and 60 ) are
especially effective in the removal of viruses and bacteria,
anti are therefore useful in hospital air conditioning. They
may also be considered for protection from nuclear fallout
and agents of chemical and biological warfare.
Electronic air cleaner efficiencies compete with the
more efficient dry filters in the submicron particle size
range. Only the self-cleaning ionizing type is suited to
high contaminant concentrations since collector plates
rapidly become less efficient as their dust load increases.
Where high particle concentrations are encountered, the
use of a viscous impingement prefilter should be
investigated.
Ionizing type precipitators are useful on high pressure
or high velocity applications for the removal of relatively
fine dirt which otherwise tends to accumulate around
discharge nozzles. Because of the relatively small
maintenance requirements of this type of filter, it may be
applied to large air tie-liveries and installations where
equipment is relatively inaccessible or where service is
infrequent or incomplete.
The characteristics of charged media air cleaners are
similar to those of medium efficiency dry filters. charged
media filters are less efficient than ionizing precipitators
but electrical failure of the associated equipment does
not completely destroy their usefulness. Operation at
relative humidities exceeding 70% may adversely affect
the dielectric properties of the medium. Individual
resistors may be provided for each filter circuit to limit the

Part 10. Air Handling Equipment | Chapter 4. Accessory Equipment

current flow thru the medium, should the medium become
damp.
Regardless of the type of filter selected, the automatic
self-cleaning feature renders servicing less dependent on
the human element and provides a relatively uniform air
resistance and air flow.
Outdoor air anti return air may he separately cleaned
with different filter types if the characteristics of the
contaminants to be removed are widely different.
Table 10 indicates the relative initial anti total annual
costs of different types of air cleaning installations.
SELECTION
Filter size is usually determined by the rated air
quantity per unit or panel as published by the
manufacturer. These rated air deliveries are established
with regard to practical air velocities as dictated by the
characteristics of the medium employed. An overload of
as much as 10% to 15% may be permissible, depending
on the medium anti on the filter construction.
Table 11 is a tabulation of typical velocities and air
resistances for various filter types. The air resistances are
base(l on clean filters. Pressure drops reflecting a
partially expended medium should be used for fan static
pressure calculations, as recommended by filter
manufacturers.
The sizes of filter units or panels are normally
standardized anti limited in number. Installations are then
built up from the basic units. Manually serviced viscous
impingement filters are usually available in sizes 20 in. x
25 in., 20 in. x 20 in., 16 in. x 25 in. and 16 in. x 20 in.
Standard thicknesses are 1 in., 2 in. and 4 in. Odd sizes
are available only at considerably higher cost unless
relatively large quantities are required.
Dry media filters are often available in only one size
from each manufacturer, but some may offer a size
selection. Charged media electronic filters are also of
limited size selection.
Ionizing type electronic cleaners and self-cleaning
filters are normally available in height increments of
several inches but in standard widths limited to two or
three sizes or combination thereof.
Some flexibility of selection may he exercised in the
use of manually serviced mechanical filters by utilizing a
‘‘V’ bank to obtain a greater ratio of filter area to crosssectional area. This method is illustrated in Fig. 64.
Desirable characteristics for viscous impingement
filter adhesive included homogeneity of film, a viscosity
relatively constant with temperature change, a resistance
to the development of mold spores and bacteria, a high
ability to wet and retain dust at all temperatures, minimal
evaporation, fire resistance, and freedom from odor.

INSTALLATION
Location
In an air conditioning system, air filters are usually
located upstream of the fan, between the cooling coil and
preheat coil, if any. This location simplifies duct and
casing design for a built-up system, avoids the net static
pressure loss associated with and acute transformation
downstream of the fan, and produces a more uniform air
distribution thru the filters. in addition, a measure of
comfort is provided in the winter for the service attendant,
and coils are protected from dust deposits and algae

Part 10. Air Handling Equipment | Chapter 4. Accessory Equipment

formation. Such a location minimizes the possibility of
introducing rain or snow, a factor of great importance in
the application of electronic air cleaners. In the case of
viscous impingement filters, adhesive temperature
variations are minimized.
If high efficiency air cleaners are used, the preferred
location is downstream of the fan. Any air leakage thin the
duct will be outward, and air cleanliness will thus be
maintained. With any high efficiency filtering device,
mechanical or electronic, located upstream of the fan, the
duct or casing between the filter and the fan should be
carefully caulked and the connections felted against
leakage.
If no preheat coil is used, an electronic cleaning
device should be located no closer to the outdoor air
intake than the height dimension of the device.
Layout
The unsatisfactory performance of air cleaning
devices can often be traced to improper installation
practices or the lack of regular maintenance. Therefore,
filter installations should be planned to meet engineering
requirement and to facilitate service.
An inspection and service area of sufficient depth
should be provided before and after the filter bank. A
minimum access of two feet for viscous impingement
filters and three feet for high efficiency dry filters is
suggested. Electronic air cleaners may require five feet
on the entering air side to permit the full opening of
swinging doors or ionizer panels. The manufacturer’s
data should be consulted for more detailed information.
Access doors should be installed in the apparatus,
upstream and downstream of the filter bank. In addition,
ladders or catwalks are required for access to filter tiers
at heights above six feet. Electric lights of the marine type
facilitate service, and are suggested on both sides of the
filter bank.
Viscous impingement and dry media unit filters are
usually removed from the entering air side of a bank.
However, they may be available for servicing from the
leaving air side if such is specifically requested. The air
how arrow should be observed in installing or replacing
progressive density filters such as the viscous

impingement type.
Duct and apparatus casing, both at the entering and
leaving air sides of the filter bank, should be (designed
and installed to insure even air distribution over the face
of the filters. This is especially important in the case of
electronic ionizing air cleaners or other low air resistance
filters. For this reason perforated plates, grilles or screens
are often installed upstream or downstream of electronic
ionizing air cleaners. Manufacturers may included such
baffling devices with the precipitator units.
Prefilters may be considered upstream of high
efficiency dry media and electronic air cleaners if high
dust or lint concentrations are present. These prefilters
also serve to distribute the air uniformly.
Proper provision should be made for the collection
and drainage of water if the filters are to be cleaned in
place by hoses or nozzles.
Outside air intakes should be located at a height and
position such that the introduction of heavy
concentrations of surface or roof dirt, automobile fumes
anti refuse is minimized. In take screens should be no
coarser than 16 mesh. Louvers should be well
constructed, particularly if the filter bank is near the
intake.
A “V” or staggered filter bank may be employed to
increase the ratio of filter surface area to cross-sectional
area. In factory-built air conditioning units with filter boxes
designed for low velocity filters, it may be necessary to
blank off uniformly a portion of the cross-sectional area if
high velocity filters are to be used. In this case, the use of
a factory-built high velocity filter box is more appropriate.
Operation of electronic filters should be dependent
upon electrical interlocks with apparatus access doors in
the interests of operator safety. These interlocks interrupt
operation as long as an access door remains open.
If sprinkler protection is required, piping must be
provided from the building sprinkler system or the city
water system. Many air cleaners feature built-in sprinkler
provisions requiring only connection.
Mechanical air filter installations should include a draft
gage or other differential pressure indicator to signal the
need for cleaning or replacing filters or to warn of the
failure of automatic filters.
MAINTENANCE
It is difficult to predict, on the basis of filter air
resistance, when a manually serviced filter will require
cleaning or replacement. Two indicators used to
determine the need for servicing are a 10% decrease in
air flow or an increase in resistance of two to three times
the initial resistance. The intervals between cleanings
vary with the application, type of filter and stage of job
installation.

Part 10. Air Handling Equipment | Chapter 4. Accessory Equipment

The rotation method of cleaning is often used,
particularly on extensive installations. Under this method,
only certain filter units are cleaned each week. Such a
practice insures a more constant work load and a more
uniform air resistance at any time.
The size of the installation dictates the most economic
manual cleaning means. Filters may be cleaned in place
with hoses or fixed nozzles on large
jobs, while smaller installations may favor the use of a
filter cleaning tank together with an appropriate number
of spare filters.
Self-cleaning filters and precipitators should be
observed for the expiration of disposable media or the
accumulation of sludge in the collecting pan. Many
manufactures provide signals for their equipment to
indicate the need for service.
Manufacturers’ recommendations regarding the
method and interval of cleaning or replacing filters should
be followed.

HEATING DEVICES
The heating devices commonly employed directly with
air conditioning systems are designed to heat air under
forced convection. They are usually located within the air
conditioning apparatus and / or ductwork.
The media used for heating include steam, hot water,
electricity and gas flame. In addition, for special
applications, glycols and hot refrigerant gas may be
used.
STANDARDS AND CODES
Methods of testing and rating forced circulation air
heating coils utilizing steam or hot water are prescribed in
ASHRAE Standard 33.
Various aspects of the construction and installation of
electric heaters are dictated by Underwriters’
Laboratories requirements. Installation of such equipment
is also governed by the National Electric Code.
The installation and piping of gas-fired duct furnaces
is prescribed by the American Standards Association
Bulletin 21.30. Installation is also influenced by
requirements of the National Board of Fire Underwriters.
The manufacture of gas-fired apparatus is directed by
standards of the American Gas Association.
The application and installation of all types of heating
devices should also conform to local codes and
regulations.
TYPES OF EQUIPMENT
Steam Coils
Steam heating coils consist of a series of tubes

connected to common headers anti mounted within a
metal casing. To insure efficient heat transfer, either plate
type or spiral fins are bonded to the tubes mechanically
or with solder. Tubing is usually constructed of copper, in
standard tubing sizes up to and including one inch OD.
Fins are often of aluminum with spacings ranging from
three to fourteen to the inch. One-row and two-row coils
are available with tubes spaced from one to three inches
on centers. Figure 65 illustrates a steam heating coil. The
offset tubes provide for changes in length due to
temperature variations.
Since the proper performance of steam coils depend
on the uniform distribution and condensation of steam in
the tube, several methods have been devised to insure
this uniformity. Individual orifices may be built into the
supply end of each tube, or distributing plates may be
installed within the steam header.
Uniform steam distribution and leaving air temperature
are also provide with the steam distributing tube type of
coil. This design features a tube within a tube, with the
inner tube perforated along its length. Steam is supplied
to the inner tube and admitted thin the orifices to the outer
or condensing tube. Condensate is collected in the return
header. A steam distributing tube coil is shown in Fig. 66,
and the principle is described in Fig. 67. In Fig. 66 note
that the tubes are pitched within the casing to promote
the rapid return of condensate.
Steam heating coils are available in various tube
lengths ranging from one foot to ten feet. Casing widths
up to four feet may be obtained for a single coil.A
Hot Water Coils
Hot water heating coils are similar in construction, size
and appearance to single tube steam coils. Although
comfort heating systems seldom require hot water coils of
more than two rows, greater depth of surface is available.
Fins are usually spaced from a minimum of seven to a
maximum of fourteen to the inch. A hot water beating coil
is shown in Fig. 68.
In order to provide optimum combinations of capacity
and water side pressure drop, various circuiting
arrangements are employed. On multiple-circuit coils,
tabulators are sometimes installed within the tubes to
produce the turbulent flow necessary for efficient heat
transfer.
Electric Heaters
Electric heating devices are commonly available in
either the open type or finned tubular type. These are
illustrated in Fig. 69 and 70. The open type consists of a
series of electrical resistance coils framed in a metal
casing and exposed directly to the air stream. The finned
tubular type of heater is made of finned steel sheaths

Part 10. Air Handling Equipment | Chapter 4. Accessory Equipment

containing resistance wire surrounded by refractory
material.
In order to achieve incremental control of heater
output, multiple electrical circuits may be obtained.
Although normal applications seldom require more than
three circuits, as many as required are possible.
Standard voltages include 115 volt, single phase, and

208 and 230 volts in single or three phase. Heaters are
also available for operation on 440 and 550 volt service.
Direct or alternating current control voltages may he
specified.
Duct Furnaces
Gas-fired furnaces are built for installation in air ducts
and in some packaged air conditioning units. Figure 71
shows a duct furnace, and Figure 51 illustrates the use of
a gas-fired furnace in a packaged unit. Such equipment
consists of a burner assembly, a heat exchanger, a
plenum and controls. Natural, manufactured or liquified
petroleum gas may be used.
APPLICATION
Heating devices are used as preheaters and
reheaters. A preheater is located upstream of the
dehumidifier in an air conditioning apparatus, and is used
either to raise the temperature of the entering air to a
temperature above freezing or to supply the heat

necessary for control of the temperature of the air leaving
the dehumidifier. Both functions are often performed by a
single heater. A reheater located downstream of the
dehumidifier is used to control the temperature of the
conditioned space when it is subjected to varying cooling
loads. Figure 36 indicates the approximate reheat
requirement dictated by a particular design relative
humidity and sensible heat ratio. A reheater may also be
used as a booster heater, compensating for wide

differences in cooling load characteristics between a
particular zone and the rest of the space conditioned by
an apparatus. If both functions are required, a central
reheater mar be used to raise the supply air temperature
to approximately room temperature or slightly higher.
Booster heaters may then be installed in the branch ducts
to the various spaces in order to provide control of room
temperature.
Steam and hot water heating coils are most commonly
employed for the applications cited. Steam coils are
normally available for steam pressures up to 200 psig
although special coils may be obtained for higher
pressures. Hot water heating coils are used on low,
mediun and high temperature hot water systems.
However, applications involving water temperatures
exceeding 300 F should be brought to the attention of the
manufacturer.

Part 10. Air Handling Equipment | Chapter 4. Accessory Equipment

Steam coils of the distributing tube type are preferred
over single tube steam coils an(l hot water (Oils for
service where freezing air temperatures are encountered
or where uniform heater leaving air temperatures are
mandatory. Single tube steam coils and hot water heaters
may, however, be used for preheat service if controlled
as described under Coil Freeze-up Protection. A
minimum entering water temperature of 150 F is
suggested for hot water preheat service.
Plate fin steam and hot water coils are preferred to
spiral for applications involving heavy concentrations of
lint since they are more easily cleaned. If spiral fin coils
are used for such applications, the widest appropriate
tube spacing should be chosen.
Where corrosive substances are present in the air,
steam or hot water, special coil materials are available.
Most steam side corrosion problems may be avoided by

the proper trapping and venting of noncondensables.
The advantages of electric heating are low initial
equipment and installation cost, a saving of floor space,
compactness, simplicity of operation and control, a fast
control response and cleanliness. Electrical facilities used
in the summer for refrigeration equipment may be used in
the winter for the heating system. At the same time
electric heating has often proven costly to operate. For
this reason it has been employed largely in mild climates
or where electrical costs are particularly low.
Since the use of electric heaters eliminates the need
for a central heating plant and piping system, electric
heating can be used to tenant areas of shopping centers,
department stores, schools, industrial facilities, banks,
motels, railroad cars and markets. Electric heaters may
be used for churches because of the short duration of
usage, the low initial cost and the quick response.
Electric heating may also be used in areas such as board
rooms or executive offices where occupancy at night or
on weekends may be common. It has been used
successfully in conjunction with self-contained air

Part 10. Air Handling Equipment | Chapter 4. Accessory Equipment

conditioning units anti as a source of auxiliary heat for
heat pump systems.
The open type of electric heater operates at a
temperature lower than that of the finned tubular heater,
and therefore exhibits a longer life. It is lighter in weight,
more rapid in response, offers less air resistance aid
tends to cycle less. The finned tubular heater is
particularity suited to applications where the heater may
be subject to mechanical injury or where an explosion
hazard exists. Stainless steel fins and sheaths are
available for high humidities or corrosive a atmospheres.
Gas-fired duct furnaces may be used for preheat and
reheat service. The advantages of such equipment are
similar to those of electric heaters. Hence, duct furnaces
may be used for similar applications in areas of relatively
high power cost. As with electric heating the problem of
coil freeze-up is not encountered. Gas-fired equipment
should never be operated in corrosive atmospheres or in
rooms where equipment should also be sufficiently
removed from acid baths or degreasing tanks.
SELECTION
The selection of a heating device involves a
consideration of the heating capacity required, the
heating medium available or required anti its
characteristics, the allowable resistance to the flow of air
an(I/or heating fluid, the entering air temperature, the air
quantity to be heated and the air velocity thru the device,
dimensional limitations, installation requirements such as
the type of control, special design requirements, and
economy.
Steam and Hot Water Coils
The capacity of a steam or hot water coil of a given
type may be increased not only by increasing the coil
surface but also by increasing the coil face velocity thru
reducing the face area. Since higher coil face velocities
result in higher air side pressure drops, the selection of a
coil surface may be more limited than at lower face
velocities. Therefore, the size and capacity of a heating
coil are interdependent, and each must be determined in
relation to the other.
Minimum coil face area is usually determined by the
design air quantity and a maximum allowable face
velocity. The dimensions of the coil may then be chosen
from among those dimensions available with the required
face area. For a given face area, coils of greater tube
length and smaller tube face are usually the least
expensive. However, space requirements may limit both
size and dimensions of a coil.
Coils are rated at face velocities of 300 to 1500 feet
per minute. The maximum face velocity should be
determined by the allowable air side pressure drop and

the ambient sound level of the space served by the coil.
Air pressure drops of 0.10 to 0.30 in. wg are suggested
for preheat applications, while reheat coil friction may
range from 0.15 to 0.35 in. wg.
Since heating coils do not condense moisture, and
since no entrainment of moisture is possible, the face
velocity of a heating coil mounted within a factory-built air
conditioning unit should not be limited to the cooling coil
face velocity.
The calculated heating load required of a coil is
usually the primary determinant of the surface selected.
Various combinations of fin spacing, tube spacing and
coil depth result in a wide variety of available surfaces.
The heat transfer capacity of a given surface varies
directly with face velocity, steam pressure, entering water
temperature or water tube velocity. ft varies inversely with
entering air temperature.
Reheat coils are usually oversized. A 15% to 25%
safety factor added to the calculated heating load
provides for a rapid morning pick-up and compensates
for duct heat losses. Steam preheat coils chosen to
operate at subfreezing air temperatures with throttling
control of steam should be undersized rather than
oversized, if the required load cannot be met exactly. This
practice reduces valve throttling at air temperatures of 25
F to 32 F, the range where excessive throttling most
usually results in the freezing of condensate in the tubes.
When using duct reheat coils for large air quantities, it
may be more economical to select a smaller coil to
handle only a portion of the air, with the remainder being
handled thru a fixed bypass around the coil. The air thin
the coil is then heated to a higher temperature so that the
mixture air is at the proper temperature. This may require
a coil with more heating surface per square foot of face
area, but in a smaller casing size. Assuming a coil face
velocity, fin spacing and rows of coil, the coil air quantity
is determined by dividing the required over-all
temperature rise by the coil temperature rise, and
multiplying by the total air quantity. The required coil face
area can be found from the coil air quantity and the
assumed coil velocity. The coil size is then selected to
match closely the calculated face area. The coil bypass is
sized as described in Part 2, and the dimensions are
chosen to coincide with the coil casing length.
Figure 72 illustrates a steam coil selection table. Coil
capacity may be expressed in terms of steam quantity
condensed, heat transferred or final air temperatures
alone. Hot water coil ratings may be similarly tabulated,
except that, at each entering air temperature, capacities
are listed for each surface at various entering water
temperatures. Another method of presenting hot water
heating coil ratings is illustrated in Fig. 73.
Heating coil performance ratings assume a rapid

Part 10. Air Handling Equipment | Chapter 4. Accessory Equipment

elimination of air and other noncondensables and a
uniform distribution of air thru the coil surface.
When steam coils are selected at face velocities
exceeding those considered standard by the
manufacturer, the amount of condensate per tube should
be checked against the maximum recommended by the
manufacturer. If the maximum allowable condensate per
tube is exceeded, excessive steam pressure drops, water
hammer and poor venting may result.
Electric Heaters
In addition to size and capacity an electric heater
selection should specify electrical characteristics and the
number of circuits required.
Electric heaters are usually chosen to fit a branch duct
of given dimensions without requiring entering and
leaving transformations. Therefore, face velocity is not the
usual determinant of coil size, although for Underwriters’
Laboratories approval a minimum face velocity must be
maintained and uniform airflow provided. This minimum
velocity is a function of entering air temperature and the
total watts per square foot of duct area. Velocities may
range up to 1800 feet per minute. Air side pressure drops
are small compared to steam and water coil pressure
drops, seldom exceeding 0.10 in. wg for an open type
coil.
All of the electrical energy used in an electric heater is
converted to heat. Thus, heater capacities in Btuh are
determined by multiplying the kilowatt rating of the heater
by 3412.
The number of circuits chosen depends on the
degree and period of heating load fluctuations. The
amount of control hunting permitted should be weighed
against the economics of purchasing and installing the
multiple circuit heater.
Duct Furnaces
Gas-fired duct furnaces are chosen according to the
output satisfying the heating load. The required air
temperature rise thru the furnace determines the air
quantity to be handled by the device and the resulting air
friction. If a greater branch duct air quantity is required, a
fixed bypass may be provided as described under Steam
and Hot Water Coils.

Atmospheric Corrections
Heating coil ratings are based on the standard atmospheric conditions of 29.92 in. Hg barometric pressure
and 70 F. For significantly different air conditions such as
at altitudes exceeding 2000 feet or at average air
temperatures above 125 F, a correction should be
applied to the required air temperature rise and the air
quantity upon which the selection is based.
In determining the coil air temperature rise required or
the heating load imposed by the admittance of air at
temperatures below the design temperatures, such as
thru ventilation or infiltration, the factor 1.08 should be
adjusted in proportion to the ratio of air densities as found
from Chart 2.
The design air quantity should be multiplied by the
density ratio in order to determine the equivalent air flow
at sea level. The adjusted air quantity and heating load
(or air temperature rise) are then used to select a coil
surface.
The coil size and face velocity are determined by the
design air flow with no correction applied. However, the
coil air side pressure drop should be corrected as
described in Part 2.
Since the capacity of an electric heater does not
depend on air quantity, no correction to the ratings is
required. However, the heating load and air friction
should be corrected as necessary.
Gas-fired duct furnaces employed at elevations
exceeding 2000 feet should be derated in output by 4%
for each 1000 feet above sea level.
COIL FREEZE-UP PROTECTION
The exposure of hot water or steam preheat and
reheat coils to subfreezing temperatures, either by
accident or intent, makes possible the freezing of water
accumulated within the tubes and thus costly damage.
The prevention of such occurrences requires
consideration of the problem in the apparatus design and
layout, in the selection of equipment, and in the choice of
control methods.
The primary requirement for positive freeze protection
is the assurance of uniform coil leaving air temperatures.

Part 10. Air Handling Equipment | Chapter 4. Accessory Equipment

Air temperature stratification may be caused by
incomplete mixing of outdoor and return air or by an
uneven temperature rise thru the coil.
Where the mixing of outdoor and return air takes place
upstream of a heating coil, mixing should be promoted by
introducing the denser cold air at the top of the plenum
and by providing as much airway length as possible. If a
steam coil is employed, the steam should be supplied
from the naturally colder side of the plenum.
If the mixing of outdoor and return air is to occur
downstream of a preheat coil, it is suggested that only the
minimum outdoor air be heated and that the maximum
outdoor air dampers be maintained closed at subfreezing
temperatures. If efficient downstream mixing has been

provided, the preheat coil may be used instead to heat
the return air to a temperature predetermined to yield the
desired mixture air temperature.
Where a steam preheat coil served by a modulating
valve tempers outdoor air, freezing of condensate in the
tubes occurs most often at entering air temperatures in
the range of 25 F to 32 F. Within this range the coil is
usually operating under a severe partial load. The
relatively small amount of steam admitted to the coil
condenses completely before the end of the tube is
reached, resulting in stratification. For this reason, if
modulating control of steam is required at subfreezing
entering air temperatures, the use of the steam
distributing tube type of coil is suggested.
Single tube steam and hot water coils may be used for
the tempering of subfreezing air, but the heating medium
should not be modulated at entering air temperatures
below 35 F. However, in cold climates such a design may
produce overheating. To provide a degree of control
while avoiding stratification, two preheat coils in series,
each furnishing a part of the required capacity and
controlled in sequence, may be employed. An alternative
method consists of the use of face and bypass dampers
controlled by a plenum thermostat. The bulb of such an
instrument should be located well downstream of the
heating coil if space permits. Otherwise, an averaging
type bulb should be used.
These same methods of obtaining control without
stratification should be considered where steam
distributing type coils of large capacity are used for
preheat service. However, rather than employing two
coils in series, it may prove economically preferable to
utilize one coil with two control valves piped in parallel.
The first valve to open may be sized to pass the minimum

Part 10. Air Handling Equipment | Chapter 4. Accessory Equipment

steam quantity necessary for even distribution of steam
thru the tubes at a signal from a two-position outdoor air
thermostat.
As mentioned previously, a minimum entering water
temperature of 150 F is suggested for tempering
subfreezing air with hot water. Uniform leaving air
temperatures should be insured as described above. In
addition, a safety control closing the outdoor air damper
at entering water temperatures below 150 F is suggested.
Another method of freeze protection is the circulation
of an inhibited glycol solution thru a water coil. A two-row
coil with a single circuit is the best protection against
stratification. The system should be designed to supply
the glycol solution to the coil at a temperature of about
150 F at peak conditions with a high temperature drop of
about 50 degrees. The steam valve to the glycol heat
exchanger is controlled by the air temperature leaving the
coil.
Another requirement for adequate freeze-up
protection of steam coils is the positive and complete
drainage of condensate from the tubes. Any type of
steam coil may be damaged if condensate is allowed to
accumulate and freeze thru poor design of the system or
coil. For this reason an ideal position for a steam preheat
coil is with the tubes vertical anti with the condensate
header at the bottom. For either horizontal or vertical air
flow, steam preheat coils installed with tubes horizontal
should be pitched downward toward the condensate
header to facilitate drainage. Many steam distributing
tube coils feature tubes internally pitched for either
horizontal or vertical air flow. In this case installation is
simplified, and the only precaution necessary is to make
sure that the condensate header is lower than the steam
header if air flow is vertical.
Positive condensate drainage is also insured by the
proper design of the condensate return system.
Adequately sized steam traps and vacuum breakers are
among the most important design considerations. Refer
to the discussion in this chapter under Layout and to Part
3
Larger heater tube diameters provide more positive
condensate drainage and more uniform leaving air
temperatures.
The outdoor air dampers of an apparatus should be
closed whenever the fan is not running. In this way the
induction of cold air by stack effect thru an inoperative
coil is minimized.
Steam preheat coil control valves, if used, should be
of the “normally open” type. Such valves should be sized
to provide the maximum required capacity at a large
pressure drop. Since valve capacity varies as the square
root of the pressure drop, the valve tends to be
undersized if steam pressure falls, and freezing is less

likely to occur within the coil.
Although occurring less frequently, the freezing of
reheat coils may be a problem, particularly if preheaters
are not employed. If such is the case, the same
provisions as described for preheat coils should be
considered if complete mixing of outdoor and return air
cannot first be guaranteed. Where face and bypass
control of a dehumidifier is employed, the preheat coil
should be located so that the air bypassed to the reheat
coil is tempered as well as the dehumidified air.
INSTALLATION
Location
In an air conditioning apparatus the preheater is
usually located between the outdoor air intake and the
filters. Reheaters are mounted downstream of the
dehumidifier coil, either within the apparatus or in the
ducts. The latter location is often chosen where more than
one control zone is served by a single air conditioning
unit.
Duct-mounted heating devices may be located
outdoors as well as indoors. Heater and duct should be
externally insulated and weatherproofed. As much of the
steam condensate return system as possible should lie
within the heated space. Terminal boxes for electric
heaters should be weatherproof.
Layout
Hot water coils and single-pass steam coils of both
the single tube and steam distributing types may be
installed with tubes horizontal or vertical and used for
vertical or horizontal air flow. Multi-pass steam coils
designed for use also with hot water are limited to
horizontal tube applications. Regardless of the
orientation, steam coils should be mounted so that the
condensate connection is below the steam connection.
Steam and water coils may be assembled in banks.
Coils so mounted should be supported individually in
angle iron frames, thus protecting the lower coils from
damage and facilitating coil removal.
Sufficient access space should be provided around a
heater to permit maintenance and removal. Connections
to ductwork should be so constructed to allow coil
removal without disturbing the duct. Duct access doors
on either side of the coil permit cleaning of the equipment
in place. Refer to Part 2 for a description of the design of
ductwork surrounding a heater.
A fixed heater bypass may be located around or
below the heating surface. A single-acting bypass
damper with blades inclined toward the leaving air side of
the heater promotes the mixing of heated and bypassed
air.
The design of hot water and steam coil piping is

Part 10. Air Handling Equipment | Chapter 4. Accessory Equipment

described in Part 3. Hot water coils should be piped so
that the water enters at the bottom connection, and coil
vents should be provided as required. In a steam coil
installation where the condensate return main is higher
than the coil steam trap, a condensate pump, lift trap or
boiler return trap should be used to move the condensate
to the main. A minimum of 18 inches should be
maintained between a steam coil condensate outlet and
the floor to provide space for traps and piping.
In the design of heater installations considerations
should be given to the prevention of air temperature
stratification. Uneven air temperature rises thru a heating
coil may result not only in coil freeze-up problems but
also in the supplying of air of non-uniform temperature to
branch ducts splitting off downstream of the heating
surface. Stratification may be minimized by the proper
design of duct splits, by the use of two coils mounted in
parallel and supplied from opposite sides of the
apparatus and, if necessary, by the use of individual duct
heaters. A horizontal supply air split is suggested with
single fan air handling units, while a vertical split is more
appropriate for multi-fan units. Other measures to reduce
stratification such as the use of steam distributing tube
coils are outlined in the section dealing with coil freeze-up
protection.

The location and layout of electric heaters and gasfired duct furnaces relative to surrounding combustible
surfaces is limited by applicable standards and codes.
When locating gas fired heaters, avoid locations
which have a positive exhaust unless provision is made to
make up air to provide for the exhaust as well as the
combustion air.
When locating electric heaters in equipment in front of
fan motors, overheating of the motors may result because
of the high temperatures that can be obtained.
Duct furnaces may be grouped in series or parallel.
Outdoor air of approximately 14 cubic feet of air per
cubic foot of gas should be provided. Flue design should
be in accordance with American Gas Association
standards.
CONTROL
The capacity of a heating coil may be varied in
accordance with the load by control of the flow of the
heating medium, by air volume control, or by air bypass
control. The control of steam or hot water flow is most
commonly employed. A multi-zone air conditioning
apparatus may utilize air bypass control. If steam is used
in this case, “on-off” control of the coil is preferred to
minimize stratification.

Part 10. Air Handling Equipment | Chapter 4. Accessory Equipment

Where a reheat coil has been selected with excess
capacity as suggested above, the use of two control
valves mounted in parallel and furnishing respectively
one-third and two-thirds of the steam required may
improve the accuracy of control at relatively low heating
loads.
Figure 74 illustrates the wiring and control of electric
heaters. The controlling instrument shown is a pneumaticelectric switch actuated by a pneumatic zone thermostat.
Electric thermostats may also be utilized. The supply duct
sail switch insures that the heaters operate only when the

fan is running. On a single-heater fan system, a thermal
switch may also be used for this purpose. Where multiple
circuit heaters are employed, individual pneumaticelectric switches and contactors are used for each circuit.
A step thermostat may be used in place of the pressure
switches.
Gas-fired duct furnaces require safety controls such
as gas valve low voltage control, a normally closed gas
valve, a high bonnet temperature cutout, a pilot safety
control, and a gas pressure regulator for other than LP
gas.

A
Access doors
Accessories
Fan
Access doors
Bearings
Drains
Isolators
Outlet dampers
Fan coil
Face and bypass damper
Filters
Humidifiers
Spray water heaters
Vibration isolation
Washing equipment
Flooding nozzles
Isolation
Spray water heater
Water cleaning device
Weirs
Accessory equipment
Air cleaners
Heating devices
Air cleaners
Application
Contaminants
Installation
Maintenance
Performance criteria
Selection
Standards and codes
Types
Dry media
Electronic
Viscous impingement
Air conditioning apparatus
Fan-coil equipment

Generalcontrol considerations
Standards and codes
Types
Coil equipment
Chart 3
Washer equipment
Chart 3
Washer equipment
Air density correction
Chart 2
Air foil blade fanFig. 5d
Application
Fan
Axial flow
Centrifugal
Atmospheric corrections
Fan
Fan0coil
Heating devices
Washer equipment
Axial flow fans
Fig. 2
Axial flow fairs.(Cont.)
Propeller (disc)
Tubeaxial
Vaneaxial4

B
Backward-curvedblade fan
Fig.
Table 1
Airfoil blade,
Fig. 5d,
Backward-inclined blade fan
Fig. 5e
Bearings
Fan
Belt drive fans
Fig. 12

Booster fans
Fig. 22

C
Centrifugal fans
Fig. 1
Table 1
Backward-curved
Forward-curved
Radial (straight)
Class of construction
Fan
Codes
Air cleaners
Air conditioning apparatus
Fans
Heating devices
Unitary equipment
Coil equipment, types, air
conditioning apparatus
Coil freeze-up protection
Fan-coil equipment
Heating devices
Unitary equipment
Contaminants
Control
Air volume
Fan-coil equipment
Unitary equipment
Washer equipment

D
Dehumidifiers
Direct drive fans
Double inlet fans
Drains
Fan

Dry media filters
Duct furnaces

E
Electric heaters
Exhaust fans

F
Face and bypass dampers
Fan arrangements
Fig. 12
Cost comparison
Table 4
Fan-coil equipment
Accessories
Application
Blow-thru unit,
Coil freeze-up protection
Control
Description
Draw-thru nit,
Installation
Location,
Multi-zone unit
Single zone unit
Spray coil section
Spray fan-coil unit
Unit selection
Fan control
Fig.20
Fan construction class
Chart 1.
Table 2.
Fan curve construction
Fan designation
Class of construction
Chart 1
Table 2
Table 3
Fan arrangements
Fig. 12
Table 4
Fan drives
Fan isolators
Fan laws
Table 5
Fan outlet velocity
Fan performance
Fan curve construction

Table 6.
laws
table 5
Fan performance in a system
Pressure considerations
System balance
Example 2
Fig. 15
Stability
Fans
Application
Axial flow
Centrifugal
Control, air volume
Fan performance
Fan performance in system
Fan selection
Location
Multiple installations
Standards and codes
Types
Fan selection
Atmospheric corrections
Accessories
Access doors
Bearings
Drains
Outlet dampers
Isolators
Variable inlet vanes
Filters
Flooding nozzles
Forward-curved blade fan
Fig 6
Table 1

Hot water coils
Humidifiers
City water spray
Humidifying pack
Steam grid
Steam pan

I
Installation
Air cleaners
Layout
Location
Fan coil equipment
Insulation
Layout
Location
Heating devices
Layout
Location
Unitary equipment
Installation (cont.)
Layout
Location
Washer equipment
Insulation
Layout
Location
Insulation
Isolators, fan

L
Location, fan

G

M

General control considerations

H
Heating coils
Heating devices
Application
Coil free-up protection
Control
Installation
Selection
Standards and codes
Types

Maintenance
Multiple installations
Recirculating fans
Fig. 21
Booster fans
Fig. 22
Return air fans
Fig. 23
Multi-zone unit

O
Outlet dampers
Fig. 19

P
Parallel fans
Propeller fan
Fig. 2
performance
Fig. 9

Air cleaners
Air conditioning apparatus
Fans
Heating devices
Unitary equipment
Fan
Heating devices
Unitary equipment

T

R
Radial blade fan
Fig. 8
Table, 1
Recirculating fans
Fig. 21
Resonance
Return air fans
Fig. 23

S
Single inlet fans
Single zone unit
Sound power level
Fig. 3
Specific speed
Fig. 4
Spray water heaters
Standards and codes

Tubeaxial fan
Fig. 2
Types of equipment
Coil equipment
Washer equipment
Unitary equipment

U
Unitary equipment
Application
Coil freeze-up protection
Control
Installation
Standards and codes
Types
Unit selection
Fan-coil
Atmospheric correction
Coil size
Unitary equipment
Atmospheric correction
Economics
Selection ratings
Washer equipment

Atmospheric correction
Spray water
Unit size

V
Vaneaxial fan
Fig. 2
Fig. 10
Fig, 11
Variable inlet vanes
Fig. 18
Vibration isolation
Viscous impingement

W
Washer equipment
Accessories
Application
Control
Installation
Unit selection
Water cleaning devices
Weirs

Fig. 1 – Centrifugal Fan
Fig. 2 – Axial Flow Fans
Fig. 3 – Sound Power Levels
Fig. 4 – Specific Speed Rangers
Fig. 5 – Fan Blades
Fig. 6 – Forward-Curved Blade Fan Performance
Fig. 7 - Backward-Curved Blade Fan Performance
Fig. 8 – Radial Blade Fan Performance
Fig. 9 – Propeller Fan Performance
Fig. 10 – Vaneaxial Fan
Fig. 11 – Axial Flow Fan Performance
Fig. 12 – Drive Arrangements
Fig. 13 – Motor Positions
Fig. 14 – Rotation and Discharge
Fig. 15 – Effect of Change in Design Conditions
Fig. 16 – Effect of Fan Curve Slope
Fig. 17 – System Insstability
Fig. 18 – Variable Inlet Vanes
Fig. 19 – Outlet Dampers
Fig. 20 – Comparison of Fan Control Methods
Fig. 21 – Recirculating Fan
Fig. 22 – Booster Fan
Fig. 23 – Return Air Fan
Fig. 24 – Single Zone Fan- Coil Unit
Fig. 25 – Multi-Zone Fan-Coil Unit

Fig. 26 – Spray Coil Section
Fig. 27 – Spray Fan-Coil Unit
Fig. 28 – Air Flow – Single Zone Unit
Fig. 29 – Air Flow –Multi-Zone Unit
Fig. 30 – Pressure Variations (DRAW-Thru Unit)
Fig. 31 – Pressure Variations (Blow-Thru Unit)
Fig. 32 – Humidification With Steam
Fig. 33 –Return Air Bypass
Fig. 34 – Required Coil Performance
Fig. 35 – Typical Cooling Coil Processes
Fig. 36 – Reheat Control Requirements
Fig. 37 – Central Station Washer
Fig. 38 – Central Station Washer (Sectional View)
Fig. 39 – High Velocity Washer
Fig. 40 – High Velocity Washer (Sectional View)
Fig. 41 – Washer Tank Arrangements (Plan View)
Fig. 42 –Two-Stage Counterflow Washer
Fig. 43 – Belt Type Water Strainer
Fig. 44 – Rotating Drum Water Strainer (Water Entering Side)
Fig. 45 – Effect of Spray Water Heater
Fig. 46 – Unitary Washer Installation
Fig. 47 – Central Station Dehumidifier Control
Fig. 48 – Typical Spray Dehumidifier Processes
Fig. 49 – Unitary Washer Control (Dehumidifier All Air)
Fig. 50 – Self-Contained Unit
Fig. 51 – Self-Contained Unit
Fig. 52 – Air-Cooled Condensing Unit
Fig. 53 – Typical Ratings (Water-Cooled Self-Contained Units)
Fig. 54 – Typical Performance Data (Unit Filter)
Fig. 55 – Viscous Impingement Filter Sector
Fig. 56 – Cleanable Viscous Impingement Filter
Fig. 57 – Automatic Viscous Impingement Filter
Fig. 58 – Dry Filter Cell With Frame

Fig. 59 – High Efficiency Dry Filter
Fig. 60 - Dry Filter Cell, Pocket Type
Fig. 61 – Automatic Dry Filter
Fig. 62 – Electronic Air Cleaner, Ionizing Type
Fig. 63 – Electronic Charged, Media Filter
Fig. 64 – Unit Filter, “V” Bank
Fig. 65 – Steam Heating Coil
Fig. 66 – Steam Distributing Tube Coil
Fig. 67 – Steam Distributing Tube Principle
Fig. 68 – Hot Water Heating Coil
Fig. 69 – Open Electric Coil Heater
Fig. 70 – Finned Tubular Electric Heater
Fig. 71 – Duct Furnace
Fig. 72 – Typical Steam Coil Ratings
Fig. 73 – Typical Hot Water Coil Rating Curves
Fig. 74 – Electric Heater Control

TABLE. 1 – CHARACTERISTICS OF CENTRIFUGAL FANS
TABLE. 2 – CLASSES OF CONSTRUCTION
TABLE. 3 – CLASSES OF CONSTRUCTION
TABLE. 4 – ARRANGEMENT COST COMPARISON
TABLE. 5 – FAN LAWS
TABLE. 6 – TYPICAL FAN TABLE
TABLE. 7 – DIRECT EXPANSION COIL RATINGS (APPARATUS DEWPOINT)
TABLE. 8 – COOLING COIL RATINGS (ENTERING WET-BULB TEMPERATURE)
TABLE. 9 – DUST CONCENTRATION RANGES
TABLE. 10 – RELATIVE AIR CLEANING COSTS∗
TABLE. 11 – OPERATING DATA

CHART. 1 – CONSTRUCTION CLASS PRESSURE LIMITS
CHART. 2 – ATMOSPHERIC CORRECTIONS
CHART. 3 – APPARATUS CLASSIFICATION
CHART. 4 – CONVERSION CHART (48F TO 60 F ADF)
CHART. 5 – CONVERSION CHART (36F TO 48 F ADF)
CHART. 6 – CHILLED WATER COIL RATINGS (APPARATUS DEWPOINT)
CHART. 7 – VIBRATION ISOLATOR DEFLECTION
CHART. 8 – SPRAY DEHUMIDIFIER RATINGS (CENTRAL STATION)
CHART. 9 – SPRAY DEHUMIDIFIER RATINGS (UNITARY TYPE)
CHART. 10 – FILTER APPLICATION

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